UTX is an X-linked tumor suppressor in T-cell leukemogenesis. (A) The UTX demethylase is within the nonautosomal region of chromosome X. Importantly, expression of this gene escapes X inactivation, and both alleles are expressed in female cells. (B) Following mutational inactivation of 1 UTX allele, female cells continue to express the wild-type allele, which appears to be sufficient for tumor suppression. By contrast, loss of the only UTX allele in male cells results in complete inactivation of this tumor suppressor, which promotes T-cell transformation.