The consistent core of clinical and haematological features observed in ATR-X patients suggests that, like the SWI2/SNF2 chromatin-remodelling protein, ATRX probably regulates transcription of a discrete set of target genes. However, although there are clearly others to be found, at present the α-globin genes remain the only confirmed targets for transcriptional regulation by ATRX. Little is currently known about the precise role of the ATRX protein during mammalian development. To investigate the role of this protein during mouse development, we generated a conditionally deleted allele of the Atrx gene in mouse embryonic stem (ES) cells, and used these cells to examine the effect of ablating expression of the full-length Atrx protein in ES cells and in mouse embryos.