A region of homology, containing the contiguous SF3A2 (formerly called SAP62) and AMH genes, exists between human chromosome 19 (HSA19p) and mouse chromosome 10 (MMU10). In a previous study it was shown that SF3A2/Sf3a2 is very highly conserved between the two species and that AMH/Amh is somewhat less conserved although both human and mouse genes encode a protein (AMH) playing the same critical role during early male sex differentiation. The close association between SF3a2/Sf3a2 and AMH/Amh was thought to maintain open chromatin in the AMH/Amh promoter region, thus facilitating the necessary precise timing of AMH/Amh expression following that of SRY/Sry at the onset of testis differentiation. Further investigation of DNA upstream of Amh has revealed that there is another gene, in close association (about 400 bp) with Sf3a2, which has significant similarities to the N-terminus of a known guanine nucleotide releasing protein (GNRP) and consequently is provisionally named GNRPx/Gnrpx. The Gnrpx-Sf3a2-Amh (GSA) locus of the mouse (MMU10) is conserved in the human (HSA19p). Mapping the Sf3a2 transcription start site eventually led us to locate and characterize its promoter. We found that Sf3a2 and Gnrpx share a bi-directional promoter, with the latter being transcribed in an antisense direction. It has now been shown by RT-PCR analysis that both Sf3a2 and Gnrpx are widely expressed and therefore are likely to be 'housekeeping' genes. GNRPx/Gnrpx messenger RNA codes for a C-terminally truncated protein (149/164 aa), which contains an as yet uncharacterized domain common to GNRPs (and related proteins) and which may therefore act as a specific antagonist of a complete GNRP protein (>1200 aa) involved in the regulation of the GTPase (G-protein/Ras) cycle.