Disclosed are a fatty acid synthase (FAS) from Schizochytrium, biologically active fragments and homologues thereof, a nucleic acid sequence encoding such FAS, fragments and homologues thereof, the gene encoding Schizochytrium FAS, host cells and organisms that recombinantly express the FAS, host cells and organisms in which the expression and/or activity of the endogenous FAS has been attenuated, and various methods for making and using any of these proteins, nucleic acid molecules, genes, host cells or organisms.