The ZFX (X-linked zinc fnger protein) gene is expressed from the inactive X chromosome and is structurally similar to its homologue on the Y chromosome, ZFY. Transcripts of ZFX and ZFY encode proteins composed of a highly acidic amino-terminal domain and a carboxy-terminal zinc-fnger motif that is commonly associated with nucleic acid-binding proteins. Conditional gene targeting studies in mouse have suggested ZFX is also required for self-renewal of embryonic and hematopoietic stem cells. ZFX has also been suggested to play a role in proliferation and expansion of B cells, and could contribute to lymphocyte homeostasis.