There is provided amicrobial cell which is capable of producing acetoacetate, 3-hydroxybutyrate and/or 3-hydroxybutyrate variants, wherein the cell is genetically modified to comprise an increased expression relative to its wild type cell of: - an enzyme E 1 capable of catalysing the conversion of acetyl-CoA to acetoacetyl-CoA; - an enzyme E 2 capable of catalysing the conversion of acetoacetyl-CoA to acetoacetate; and -an enzyme E 3 capable of catalysing the conversion of acetoacetate to 3-hydroxybutyrate and/or variants thereof.