TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Cyanobacterium: The genome harbors more than one copy of the first essential or conditionally essential gene and where all copies of the first essential gene carry at least one gene inactivation. The first gene inactivation comprises an at least partial deletion of the first essential or conditionally essential gene. The first gene inactivation comprises a complete deletion of the first essential or conditionally essential gene. The first extrachromosomal plasmid comprises an endogenous plasmid of the cyanobacterium into which the first essential or conditionally essential gene and the at least one first production gene were integrated. The cyanobacterium is Synechococcus PCC7002 and the endogenous plasmid is selected from pAQ1, pAQ3, pAQ4 and the plasmid pAQ5. At least one production gene comprises at least two first and second production genes coding for separate first and second production enzymes, which produce the first chemical compound. The first and second production genes are transcriptionally controlled by different promoters. The essential or conditionally essential gene is transcriptionally controlled by a different promoter than the at least one first production gene. The essential or conditionally essential gene and the at least one first production gene are controlled by different inducible promoters. The first gene essential or conditionally essential for the cyanobacterium is selected from genes encoding enzymes for the amino acid metabolism, nucleic acid metabolism, carbon metabolism, nitrogen, sulfur or phosphorus metabolism of the cyanobacterium, genes encoding essential RNA molecules, such as ribosomal RNA (rRNA), transfer RNA (tRNA), resistance conferring genes or their combination. The first essential gene is selected from pyrF, rbcLXS, leuB, petB/petD, psaC, hisB, atpB/atpE or their combination or homologous or analogous first essential genes. The first conditionally essential gene is selected from narB, ziaA, smtA, nrsRS, corT, nrsA, arsBHC or their combination or homologous or analogous first essential genes. The first essential gene encodes one of the following: a first essential biocatalyst which is involved in the production of a first essential factor, which cannot promote complete genetic segregation of the cyanobacterium with regard to the first gene inactivation if present in the growth medium of the cyanobacteria or the first essential factor itself, which cannot promote complete genetic segregation of the cyanobacterium with regard to the first gene inactivation if present in the growth medium of the cyanobacteria. The first essential or conditionally essential biocatalyst is selected from enzymes for the nucleic acid metabolism, enzymes conferring resistance to metal or semi-metal ions and enzymes for the amino acid metabolism. The first essential or conditionally essential biocatalyst is selected from pyrF, rbcLXS operon (RubisCO), and leuB or their combination. At least one first production gene is under the transcriptional control of an inducible or constitutive promoter. The promoter is inducible by nutrient starvation and light exposure. The inducible promoter is selected from ntcA, nblA, isiA, petJ, petE, ggpS, psbA2, psaA, sigB, IrtA, htpG, nirA, hspA, clpB1, hliB, rbc, and crhC. The inducible promoter is selected from nirA, petJ, petE, or their combination. The constitutive promoter is from RubisCO. At least one first production gene encodes an enzyme for ethanol production. The enzyme for ethanol production is selected from Adh, Pdc, AdhE or their combination. The cyanobacterium further comprises a second gene inactivation in a second essential or conditionally essential gene in the genome of the cyanobacterium, where the second essential or conditionally essential gene is different from the first essential or conditionally essential gene, and at least one second production gene different from the first production gene, where the second essential or conditionally essential gene and the second production gene are included on either the first extrachromosomal plasmid or on a second extrachromosomal plasmid. The second production gene encodes an enzyme for the production of a second chemical compound, which is selected from alcohols, alkanes, polyhydroxyalkanoates, such as polyhydroxybutyrate (PHB), fatty acids, fatty acid esters, carboxylic acids, such as amino acids, terpenes and terpenoids, peptides, polyketides, alkaloids, lactams, such as pyrrolidone, and ethers, such as tetrahydrofuran (THF) or their combination. The second production gene encodes an endogenous enzyme of the cyanobacterium, the expression of the endogenous enzyme resulting in an increased rate of production of the first chemical compound compared to the respective cyanobacterium harboring the first production gene, but lacking the second production gene. The endogenous enzyme is selected from enzymes of the glycolysis pathway, Calvin cycle, intermediate steps of metabolism, amino acid metabolism, the fermentation pathway and the citric acid cycle. The first, and if present also second extrachromosomal plasmid, is replication competent. The first gene inactivation comprises a partial deletion of the first essential or conditionally essential gene and where the first extrachromosomal plasmid harbors a gene which is homologous to the first essential or conditionally essential gene inactivated by the first gene inactivation. The first gene inactivation comprises a complete deletion of the first essential or conditionally essential gene and where the first extrachromosomal plasmid harbors the same first essential or conditionally essential gene inactivated by the first gene inactivation. The first essential or conditionally essential gene included on the first extrachromosomal plasmid encodes an essential or conditionally essential biocatalyst, which is homologous or analogous to the biocatalyst encoded by the first essential or conditional essential gene in the genome of the cyanobacterium harboring the first gene inactivation. The cyanobacterium is selected from Synechocystis, Synechococcus, Anabaena, Chroococcidiopsis, Chloreogloepsis, Cyanothece, Lyngbya, Phormidium, Nostoc, Spirulina, Arthrospira, Cyanobacterium, Trichodesmium, Leptolyngbya, Plectonema, Myxosarcina, Pleurocapsa, Oscillatoria, and Pseudanabaena. Ethylene producing cyanobacterium, where the at least one first production gene encodes an enzyme for ethylene formation. Isoprene producing cyanobacterium, where the at least one first production gene encodes an enzyme for isoprene formation. Preferred Method: In the method for producing a first chemical compound, in step (a), the genetically enhanced cyanobacteria are subjected to light, in particular sunlight and to carbon dioxide (CO2). The cyanobacterium harbors a first gene inactivation in a first conditionally essential gene and in step (a), the cyanobacteria are cultured under a condition rendering the first conditionally essential gene an essential gene. For the production of ethanol, cyanobacteria are cultured harboring a first production gene encoding at least one enzyme for ethanol production. At least one enzyme for ethanol production is selected from Adh, Pdc, AdhE or their combination. Preparation (claimed): Producing genetically enhanced cyanobacteria comprises (a) transforming the cyanobacterium by introducing a first gene inactivation into a first essential or conditionally essential gene of the cyanobacterium, and (b) introducing the first extrachromosomal plasmid harboring the first essential or conditionally essential gene and the at least one first production gene into the genetically enhanced cyanobacterium. Step (a) comprises replacing at least a part of the first essential or conditionally essential gene of the cyanobacterium with a recombinant nucleic acid sequence, thus creating the first gene inactivation. Step (a) comprises the following substeps: (i) (a1) transforming the cyanobacteria with a first recombinant nucleic acid sequence, where the nucleic acid sequence comprises a first selectable gene conferring resistance to a selectable marker and a second counterselectable gene conferring sensitivity to a counterselectable marker; (a2) selecting for transformed cyanobacteria by subjecting the cyanobacteria to the selectable marker; (a3) transforming the cyanobacteria obtained from step (a2) with a second recombinant nucleic acid sequence lacking the first selectable and second counterselectable gene by replacing at least a part of the first recombinant nucleic acid sequence, thus creating transformed cyanobacteria lacking a functional first selectable and functional second counterselectable gene; (a4) selecting for transformed cyanobacteria from step (a3) via subjecting the cyanobacteria to the counterselectable marker; and (ii) introducing the extrachromosomal plasmid into the cyanobacteria obtained from step (a4). The second counterselectable gene is selected from sacB, tetAR, rpsL, pheS, thyA, lacy, gata-1, and ccdB or their combination. The first selectable gene confers biocide resistance. In substeps (a1) and (a3), the cyanobacteria are transformed via homologous recombination. The genome of the cyanobacteria harbors more than one copy of the first essential gene, the essential gene encoding a biocatalyst for the production of a first essential factor, which cannot promote complete genetic segregation of the cyanobacterium with regard to the first gene inactivation if present in the growth medium and which includes the following substeps: (i) (a'1) first gene inactivations are created in not all copies of the first essential gene by replacing at least parts of the first essential gene by a first recombinant nucleic acid, (a'2) selecting for the cyanobacteria obtained in substep (a'1) by subjecting the cyanobacteria to the first selectable marker, Followed by step (ii) of introducing the first extrachromosomal plasmid harboring the first essential or conditionally essential gene and the at least one first production gene into the genetically enhanced cyanobacteria of substep (a'2), (a'3) subjecting the cyanobacteria from step (ii) to concentrations of the first selectable marker higher than the concentration used in substep (a'2), (a'4) transforming the cyanobacteria obtained from step (a'3) with the second recombinant nucleic acid, (a'5) selecting for cyanobacteria obtained in substep (a'4) by subjecting the cyanobacteria to the counterselectable marker. The essential biocatalyst is selected from pyrF, rbcLS and leuB or their combination. EXAMPLE - No suitable example given.