Deficiency of steroid sulphatase (STS) is associated with ichthyosis, with failure of the placental production of oestriol in late pregnancy and with difficulties in childbirth. The STS gene has been localised by deletion mapping to the distal tip of the snort arm of the X chromosome, and is of interest in that it appears to escape X-inactivation. We have constructed an X-specific DNA library and screened it for single copy DNA sequences which lie at the distal end of Xp. The sequence GMGX9 was found to map in the interval Xp22.3-pter and to detect a frequent HindIII polymorphism. We have used GMGX9 in linkage studies in families with classical X-linked ichthyosis and this has not only shown tight linkage with STS deficiency but has also revealed that the sequence is deleted in affected males in eight of nine families. GMGX9 is present in all of 26 normal male individuals so far examined. Our findings suggest that a high proportion of the mutations at the STS locus leading to enzyme deficiency are deletions, presumably generated by unequal cross-over events in female meiosis or by illegitimate X-Y interchange in male meiosis.