The major product of the XLR (X-chromosomal, lymphocyte-regulated) locus is found to be a 30-kD nuclear protein with a relatively short (t1/2 approximately equal to 2 h) half-life. Together with its stage- and tissue-specific pattern of expression, this suggests a role for this protein in the regulation of differentiation in T and B lymphocytes. Interestingly, the XLR protein almost completely leaches out of the nucleus after lysis of cells in low salt buffer, but is stabilized in that location by metal cations, particularly Zn++. This stabilization is reversible by chelating agents (o-phenanthroline, EDTA) which also release a number of other polypeptides in addition to XLR. These results suggest that XLR represents a novel class of nuclear proteins, and that cations such as zinc may play a role in the localization of these proteins in the nucleus.