An expression cassette, operable in a recombinant host, comprising a heterologous DNA coding sequence encoding a protein, which is functional, alone or in cooperation with one or more additional proteins, of catalyzing an oxidation step in the biological pathway for conversion of cholesterol into hydrocortisone, which step is selected from the group consisting a of: the conversion of cholesterol to pregnenolone; the conversion of pregnenolone to progesterone; the conversion of progesterone to 17null-hydroxy-progesterone; the conversion of 17null-hydroxyprogesterone to cortexolone; the conversion of cortexolone to hydrocortisone, and the corresponding control sequences effective in said host.