TECHNOLOGY FOCUS - BIOLOGY - Preferred Components: The stronger promoter is a constitutive promoter. The promoter sequence is HXT7, shortened HXT7, PFK1, FBA1, TPI1, PGK1, PMA1, ADH1 or TDH3. The 2-keto acid decarboxylase is an enzyme of Pdc1 (SEQ. ID. No. 14), Pdc5 (SEQ. ID. No. 16), Pdc6 (SEQ. ID. No. 18), Aro10 (SEQ. ID. No. 20) or THi3 (SEQ. ID. No. 22). The alcohol dehydrogenase is an enzyme of Adh1 (SEQ. ID. No. 24), Adh2 (SEQ. ID. No. 26), Adh3 (SEQ. ID. No. 28), Adh4 (SEQ. ID. No. 30), Adh5 (SEQ. ID. No. 32), Adh6 (SEQ. ID. No. 34), Adh7 (SEQ. ID. No. 36), Sfa1 (SEQ. ID. No. 38) or Ypr1 (SEQ. ID. No. 40). The over-expressed gene is over-expressed in a codon-optimized variant manner, where the codon-optimization is aligned at codon-usage of the highly expressed glycolytic gene of the yeast. The genes of all enzymes, which are involved in the conversion of pyruvate to isobutanol, are over-expressed and all the genes are over-expressed in the codon-optimized variant. The cell expresses acetohydroxy acid reductoisomerase, which exhibits an increased specificity for NADH against NADPH. The NADH-preferring acetohydroxy reductoisomerase is a mutant variant of Ilv5-enzyme of the yeast. The NADH-preferring alcohol dehydrogenase of the yeast is over-expressed and simultaneously the isobutyraldehyde is converted to isobutanol. The cell additionally expresses a phosphorylating glyceraldehyde-3-phosphate dehydrogenase that exhibits an increased specificity for NADP+ against NAD+. The NADP-preferring glyceraldehyde-3-phosphate dehydrogenase is heterologous for host cell. The NADP-preferring glyceraldehyde-3-phosphate dehydrogenase is coded by one, two or all three mutated alleles of TDHI-3 (SEQ. ID. No. 41), (SEQ. ID. No. 43), (SEQ. ID. No. 45) gene of the yeast. The NADP-preferring glyceraldehyde-3-phosphate dehydrogenase is expressed in a yeast cell that exhibits none or reduced expression or activity of the NAD-glyceraldehyde-3-phosphate dehydrogenase. The NADPH-preferring glyceraldehyde-3-phosphate dehydrogenase is expressed and simultaneously the isobutyraldehyde is converted to isobutanol. The enzyme acetolactate synthase, acetohydroxy acid reductoisomerase and dihydroxy acid dehydratase are localized in the cytosol of the cell. In the yeast cell: additionally Ilv6-protein (YCL0O9C) (SEQ. ID. No. 4) is over-expressed in the same cell compartment as that of Ilv2; and additionally the expression of the gene PDC1 (SEQ. ID. No. 13), PDC5 (SEQ. ID. No. 15) and PDC6 (SEQ.ID. No. 17) and/or the activity of the coding enzyme is reduced and/or interrupted. The yeast cell is Pichia, Candida, Hansenula, Kluyveromyces, Yarrowia or Saccharomyces (preferably Saccharomyces cerevisiae), where the sequences not defined here may be found at ftp://ftp.wipo.int/pub/published_pct_sequences/publication.    TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preparation: No preparation method is given. Preferred Process: The host cell is contacted with the carbon source in the culture medium.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The fermentable carbon source is 3-6C carbon source including monosaccharide, oligosaccharide or polysaccharide (preferably glucose, fructose, mannose, galactose, saccharose, maltose, xylose or arabinose). EXAMPLE - No suitable example given.