The serine hydroxymethyltransferase (SHMT) gene family is composed of three distinct loci. The cytosolic (cSHMT) and mitochondrial (mSHMT) genes constitute the functional members of the gene family, while the third member, SHMT-ps1, is a processed pseudogene descended from cSHMT. PCR analysis of 38 primate and nonprimate mammal species indicates that the reverse transcription event that gave rise to SHMT-ps1 might have occurred after the divergence of the primates from the rest of the mammals. In addition, direct sequencing of primate PCR products has revealed several features--including two deletions, an insertion, and two single base mutations--that are unique to specific phylogenetic branches of the order Primates. These unique features make the SHMT-ps1 locus a useful marker in molecular studies of the primates.