A form of the human skin disease, ichthyosis, results from a mutation at the steroid sulphatase locus (STS) on the X chromosome. This locus appears to escape inactivation in the XX female, resulting in the expression of two doses of the STS gene (Shapiro et al. 1978; Crawfurd, 1982). The scurfy mutation in the mouse is thought to be homologous to the human disease (McKusick, 1978), and so should also be due to a steroid sulphatase (STS) deficiency. Our findings in male and female mice suggest that, in contrast to the human, the murine ‘STS’ locus is subject to X chromosome inactivation. However, another interpretation of the results is possible, namely that STS may be coded for by an autosomal gene.