Mutations in the ATRX gene are associated with an X‐linked mental retardation (XLMR) syndrome most often accompanied by α-thalassaemia (ATR-X syndrome). The ATRX gene encodes a predicted protein of 280 kDa featuring a PHD zinc finger motif and an ATPase/helicase domain of the SWI/SNF type; the vast majority of mutations in the ATRX gene fall within these two motifs. Although these domains are suggestive of a role for ATRX in transcriptional regulation by affecting chromatin structure and/or function, the precise cellular role of the ATRX protein remains undefined. Using indirect immunofluorescence and biochemical frac‐ tionation, we demonstrate that the ATRX protein has a punctate nuclear staining pattern and that it is tightly associated with the nuclear matrix at interphase. At the onset of M phase, the ATRX protein was associated mainly with condensed chromatin. The association of the ATRX protein with chromosomes at mitosis is concomitant with phosphorylation of the protein and its association with heterochromatin protein 1α (HP1α). The phosphorylation-dependent changes in localization between the nuclear matrix and condensed chromatin are consistent with a dual role for ATRX, possibly involving gene regulation at interphase and chromo‐ somal segregation at mitosis.