X-linked ichthyosis is an X-linked recessive disorder of abnormal cornification caused by a deficiency of steroid sulfatase, an enzyme that is needed for normal desquamation of the stratum corneum. It is classically characterized by dirty-appearing scale on the sides of the neck ("dirty neck") and the periumbilical area. The remainder of the trunk and extremities are also involved, with classic sparing of the face, palms, and soles, as well as the popliteal and antecubital fossae.

This condition is exclusively seen in males, appears before 3 months of age, and persists for life. During the first few weeks of life there is desquamation of large, loosely adherent, translucent scales followed by the development of tightly adherent, dark brown scales. The disease phenotype ranges from absent to marked and diffuse scaling. It is worse in low-humidity climates.

The disease may be diagnosed prenatally due to a low estriol on a maternal triple screen and confirmed by fluorescence in situ hybridization (FISH). Women with affected fetuses may experience lack of spontaneous labor and prolonged labor, possibly requiring cesarean delivery or vacuum-assisted delivery due to a lack of steroid sulfatase in the placenta, leading to decreased levels of estrogen and insufficient dilation of the cervix.