TECHNOLOGY FOCUS - BIOLOGY - Preparation (Claimed): Making the recombinant microorganism, comprises transforming a parental cell with one or more vectors for expressing a polypeptide having beta-ketothiolase activity, acetyl-coenzyme A acetyltransferase activity, 3-hydroxybutyryl-coenzyme A dehydrogenase activity, crotonase activity, trans-2-enoyl-coenzyme A reductase activity and/or bifunctional aldehyde/alcohol dehydrogenase activity. Preferred Components: The recombinant microorganism comprises: a heterologous polypeptide having beta ketothiolase activity and a heterologous polypeptide having acetyl-coenzyme A acetyltransferase activity; heterologous expression or elevated expression of a polypeptide having trans-2-enoyl-coenzyme A reductase activity, as compared to a parental microorganism, where the recombinant microorganism produces a metabolite comprising butyryl-coenzyme A from a substrate that includes crotonyl-coenzyme A; heterologous expression or elevated expression of a polypeptide having acetyl-coenzyme A acetyltransferase activity, as compared to a parental microorganism, where the recombinant microorganism produces a metabolite comprising acetoacetyl-coenzyme A from a substrate comprising acetyl-coenzyme A; heterologous expression or elevated expression of a polypeptide having hydroxybutyryl-coenzyme A dehydrogenase activity, as compared to a parental microorganism, where the recombinant microorganism produces a metabolite comprising 3-hydroxybutyryl-coenzyme A from a substrate comprising acetoacetyl-coenzyme A; heterologous expression or elevated expression of a polypetpide having crotonase activity, as compared to a parental microorganism, where the recombinant microorganism produces a metabolite comprising crotonyl-coenzyme A from a substrate comprising 3-hydroxybutyryl-coenzyme A; heterologous expression or elevated expression of a polypeptide having aldehyde/alcohol dehydrogenase activity, as compared to a parental microorganism, where the recombinant microorganism produces a metabolite comprising buteraldehyde from a substrate comprising butyryl-coenzyme A; and elevated expression of a polypeptide having beta-ketothiolase activity, as compared to a parental microorganism, where the recombinant microorganism produces a metabolite comprising 3-ketohexanoyl-coenzyme A from substrates comprising butyryl-coenzyme A and acetyl-coenzyme A. The trans-2-enoyl-coenzyme A reductase (Ter) is derived from a Treponema denticola or F.succinogenes. The polypeptide having trans-2-enoyl-coenzyme A (Ter) polypeptide activity is at least 50% identical to a sequence of SEQ ID NO: 68 (comprising a 60 amino acid sequence fully defined in the specification), SEQ ID NO: 69 (comprising a 59 amino acid sequence fully defined in the specification), SEQ ID NO: 70 (comprising a 59 amino acid sequence fully defined in the specification), or SEQ ID NO: 71 (comprising a 54 amino acid sequence fully defined in the specification). The Ter comprises an M11K substitution. The polypeptide having acetyl-coenzyme A acetyltransferase activity is encoded by a polynucleotide having at least 50% identity to a sequence of SEQ ID NO: 29 (comprising a 1187 base pair sequence fully defined in the specification). The polypeptide having acetyl-coenzyme A acetyltransferase activity is encoded by an atoB gene or its homolog, or a fadA gene or its homolog. The atoB gene or fadA gene is derived from the genus Escherichia. The Escherichia is E.coli. The polypeptide having hydroxybutyryl-coenzyme A activity is encoded by a polynucleotide having at least 50% identity to a sequence of SEQ ID NO: 35 (comprising a 849 base pair sequence fully defined in the specification), SEQ ID NO: 72, 73, 74, 75, 76 or 77. The hydroxybutyryl-coenzyme A dehydrogenase is encoded by an hbd gene or its homolog or a paah1 gene or its homolog. The hbd gene is derived from a microorganism comprising Clostridium acetobutylicum (preferred), Clostridium difficile, Dasytricha ruminantium, Butyrivibrio fibrisolvens, Treponema phagedenis, Acidaminococcus fermentans, Clostridium kluyveri, Syntrophospora bryanti, or Thermoanaerobacterium thermosaccharolyticum. The polypeptide having crotonase activity is encoded by a polynucleotide having at least 50% identity to a sequence of SEQ ID NO: 51 (comprising a 786 base pair sequence fully defined in the specification). The crotonase is encoded by a crt gene or its homolog. The crt gene is derived from a microorganism comprising Clostridium acetobutylicum (preferred), Butyrivibrio fibrisolvens, Thermoanaerobacterium thermosaccharolyticum, or Clostridium difficile. The polypeptide having aldehyde/alcohol dehydrogenase activity is encoded by a polynucleotide having at least 50% identity to a sequence of SEQ ID NO: 67 (comprising a 2581 base pair sequence fully defined in the specification). The polypeptide having aldehyde/alcohol dehydrogenase is encoded by an aad gene or its homolog, or an adhE2 gene or its homolog. The aad gene or adhE2 gene is derived from Clostridium acetobutylicum. The polypeptide having beta-ketothiolase activity is encoded by a bktB gene or its homolog. The bktB is derived from R.eutropha. The bktB gene or its homolog comprises a sequence having at least 50% identity to SEQ ID NO: 37 (comprising a 1185 base pair sequence fully defined in the specification). The microorganism comprises a knockout in a gene comprising ldhA, adhE and/or frdBC. The microorganism expresses polypeptides having beta-ketothiolase activity, acetyl-coenzyme A acetyltransferase activity, 3-hydroxybutyryl-coenzyme A dehydrogenase activity, crotonase activity, trans-2-enoyl-coenzyme A reductase activity and aldehyde/alcohol dehydrogenase activity. The microorganism comprises an atoB, BktB, hbd or paaH1, crt and a ter gene, where sequences not defined here may be found at ftp://ftp.wipo.int/pub/published-pct-sequences/publication. EXAMPLE - No suitable example given.