Servicio de Genética, Hospital General de México.
X-linked recessive ichthyosis (XLI) is an inherited inborn error of metabolism due to steroid sulfatase (STS) deficiency. The STS activity was studied in 13 families that were referred to the Genetic Department, General Hospital of Mexico City, as being affected by ichthyosis. The study was specially focused on five apparently on familial cases and their mothers, in order to identify carrier status and provide adequate genetic counseling. STS activity was determined in leucocytes using 7-[3H]-dehydroepiandrosterone sulfate as substrate. None of the XLI patients showed STS activity (pmol/mg protein/h), four mothers had an activity compatible with a carrier state (0.19 +/- 0.02 vs 0.66 +/- 0.14 males or 0.90 +/- 0.30 females pmol/mg protein/h, p < 0.001) and only one mother showed a normal pattern, indicating that her son had a de novo mutation. It is important to determine the STS activity in the propositus mother of apparently non familial cases of XLI to identify the carrier state and provide and accurate genetic counseling, as most of these seem to correspond to inherited cases.