We have identified and characterized a cDNA encoding human Fas associated factor 1 (hFAF1) cDNA and a shorter form of hFAF1 cDNA [hFAF1(s)] with a 456 bp internal in-frame deletion from a human HeLa cDNA library. The nucleotide sequences of hFAF1 and hFAF1(s) were identical except for the deletion. GST-hFAF1 fusion protein bound to the in vitro translation product of Fas. The N-terminal region (amino acid 1~201) including the upstream ubiquitin homology domain of hFAF1 could bind with the death domain of Fas unlike that of qFAF1 whose binding region with Fas could not be determined. However hFAF1 did not bind to the death domain of Fas mutant, lpr(cg). hFAF1 was expressed abundantly in testis, skeletal muscle, and heart as 2.8 kb mRNA. Polyclonal antibody against hFAF1 detected 74 kD protein, a deduced protein size from the ORF and 40 kD protein in some cell lines.