Human Fas associated factor 1 protein (hFAF1) is involved in the positive regulation of Fas signaling even though it can not initiate the signal for itself. By chromosomal assignment using somatic cell hybrids (CASH), the hFAF1 gene was located on human chromosome 1 between markers D1S443 and D1S197. The hFAF1 gene was mapped to human chromosome band 1p32 by FISH utilizing a genomic PAC clone containing the gene. In genomic Southern analysis using hFAF1 cDNA as a probe, several bands appeared in three different restriction enzyme digestions. The single band appearance in FISH analysis compared to several bands in Southern blots implies that the hFAF1 gene would be rather big or that an additional hFAF1 gene isotype(s) might be present in close vicinity.