The first branch is catalyzed in "E. coli" by enzymes encoded by "epd", "pdxB", "serC" and "pdxA". These share mechanistical similarities and homology with the three enzymes in serine biosynthesis ("serA" (homologue of "pdxB"), "serC", "serB" — however, "epd" is a homologue of "gap"), which points towards a shared evolutionary origin of the two pathways. In several species there are two homologues of the "E. coli" "serC" gene, generally one in a ser operon ("serC"), and the other in a pdx operon, in which case it is called "pdxF".