TECHNOLOGY FOCUS - BIOLOGY - Preferred Components: The precursor thioesterase comprises at least one sequence, which is: an amino acid sequence of formula (Gly-Asp-Ser-Leu-Xaa-Xaa-Xaa-Xaa-Xaa-Met) (I) (SEQ ID NO: 28); an amino acid sequence of formula (Val-Xaa-Xaa-Gly-Xaa-Asn-Asp-Xaa-Leu) (II) (SEQ ID NO: 29); and/or an amino acid sequence of formula (Asp-Xaa-Xaa-His-Pro-Xaa-Xaa-Xaa-Xaa-Xaa-Xaa-Xaa-Ile) (III) (SEQ ID NO: 30). The precursor thioesterase: has 30% (preferably 95%) sequence identity to the fully defined sequence of 183 amino acids of SEQ ID NO: 31 (derived from Escherichia coli), as given in the specification; and is encoded by a polynucleotide having 30% (preferably 95%) sequence identity to the fully defined sequence of 552 nucleotide bases of SEQ ID NO: 32 (derived from E. coli), as given in the specification. The altered property is: an altered substrate specificity in vitro and/or in vivo, where the altered substrate specificity is an increased activity with respect to 10C, 12C or 14C substrates, or with respect to ester substrates or thioester substrates; an altered proportional yield of fatty esters versus other fatty acid derivatives in vitro and/or in vivo, where the altered proportional yield is an increased or a decreased proportional yield of fatty esters versus free fatty acids in vitro and/or in vivo; an increased yield of fatty acid derivatives in vitro and/or in vivo; or an increased or a decreased proportional yield of short-chain fatty acid derivatives versus other fatty acid derivatives in vitro and/or in vivo. The short-chain fatty acid derivatives are 8C-14C fatty acids derivatives (a1). The other fatty acid derivatives are 15C-20C fatty acid derivatives (a2). (a1) and (a2) are free fatty acids, fatty esters, fatty aldehydes or fatty alcohols. The fatty acid derivative enzyme is an acyl-CoA synthase, an ester synthase, an alcohol acyltransferase, an alcohol dehydrogenase, an acyl-CoA reductase, a fatty acid decarboxylase, a carboxylic acid reductase, a fatty alcohol acetyl transferase, a decarbonylase and/or a fatty alcohol forming acyl-CoA reductase. In formula (I),Xaa = amino acid, where the Asp residue is substituted with Asn or Thr, the Leu residue is substituted with Cys or Gln, and the Met residue is substituted with Cys, Asp, Leu, Asn, Thr or Val.In formula (II),Xaa = amino acid, where the Val residue is substituted with Leu, the Asn residue is substituted with Val, Leu, Cys, Ala, Gly, His, Ile, Thr or Trp, the Asp residue is substituted with Glu, and the Leu residue is substituted with Ile, Trp, Phe, Thr, Met, Ala, Glu, Asn or Val.In formula (III),Xaa = amino acid, where the Pro residue is substituted with Gly, Ala, Phe, Leu, Ser or Val, and the Ile residue is substituted with Leu or Val.    TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preparation: No preparation method is given. Preferred Components: In (B1), (Ia) is integrated into the chromosome of the host cell.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The fuel composition further comprises: a petroleum-based fuel; a synthetic fuel; and at least one of cetane number improvers, cloud point lowering additives, detergents, dispersants, anti-wear agents, lubricity improvers, stabilizers, antioxidants, corrosion inhibitors, biocides, metal deactivators, viscosity improvers, friction modifiers, foam inhibitors or seal fixes. EXAMPLE - No suitable example given.