YY1 is a ubiquitous and multifunctional zinc-finger transcription factor involved in the activation or repression of a large number of genes that are critical for basic biological processes of cell growth, development, differentiation, cell cycle and even programmed cell death (apoptosis). YY1 is an essential protein; deletion of YY1 leads to lethality in mouse embryos and disruption of one allele causing developmental defects.