DFFRY (the Y-linked homologue of the DFFRX Drosophila fat-facets related X gene) maps to proximal Yq11.2 within the interval defining the AZFa spermatogenic phenotype. The complete coding region of DFFRY has been sequenced and shows 89% identity to the X-linked gene at the nucleotide level. In common with DFFRX , the potential amino acid sequence contains the conserved Cys and His domains characteristic of ubiquitin C-terminal hydrolases. The human DFFRY mRNA is expressed in a wide range of adult and embryonic tissues, including testis, whereas the homologous mouse Dffry gene is expressed specifically in the testis. Analysis of three azoospermic male patients has shown that DFFRY is deleted from the Y chromosome in these individuals. Two patients have a testicular phenotype which resembles Sertoli cell-only syndrome, and the third diminished spermatogenesis. In all three patients, the deletions extend from close to the 3' end into the gene, removing the entire coding sequence of DFFRY. The mouse Dffry gene maps to the Sxrb deletion interval on the short arm of the mouse Y chromosome and its expression in mouse testis can first be detected between 7.5 and 10.5 days after birth when type A and B spermatogonia and pre-leptotene and leptotene spermatocytes are present.