X-linked ichthyosis is a disorder in which the skin cells are produced at a normal rate but they do not separate properly at the surface of the stratum corneum (the outermost layer of the skin). This slows the rate of shedding of the skin cells, resulting in a build-up of scales. The scales of X-linked ichthyosis are often dark and usually cover only a portion of the body. Typically, the trunk and back of the neck are more likely to be affected. Scales are usually not found on the face, scalp, palms of the hands, and soles of the feet. X-linked ichthyosis mostly affects males. It is usually caused by a genetic change or deletion in the STS gene and is inherited in an X-linked recessive manner. In rare cases, the ichthyosis can be part of a genetic syndrome in which other parts of the body are affected.