The ZFX (X-linked zinc finger protein) gene is expressed from the inactive X chromosome and is structurally similar to its homologue on the Y chromosome, ZFY (1). Transcripts of ZFX and ZFY encode proteins composed of a highly acidic amino-terminal domain and a carboxyterminal zinc-finger motif that is commonly associated with nucleic acid-binding proteins (2). Both ZFY and ZFX are probable transcriptional activators and may function in sex determination (3). Multiple alternatively spliced transcript variants of ZFX, encoding different isoforms, have been identified and may be functionally distinct (2). Conditional gene targeting studies in mouse have suggested ZFX is also required for self-renewal of embryonic and hematopoietic stem cells (4). ZFX has also been suggested to play a role in proliferation and expansion of B cells, and could contribute to lymphocyte homeostasis (5).