is typically termed the "sex chromosomes". Females have two X chromosomes and males have one X and one Y chromosome. Therefore, in normal individuals, males carry a single copy of the STS gene and females carry two copies. This gene partially escapes X-inactivation and females normally express higher amounts of the STS enzyme than males. XLI can occur through new deletions or mutations of the STS gene but is more commonly inherited from a carrier mother. A hemizygous deletion or mutation of the STS gene in a male results in complete absence of enzyme activity, while a female carrier of a mutation or deletion is heterozygous and still has a normal copy of the STS gene. Female carriers of an STS deletion or mutation still express the STS enzyme, although with decreased enzyme activity. For this reason, XLI most commonly affects males, although individuals with numeric abnormalities of the sex chromosomes (45,X and 47,XXY) who also carry STS deletions or mutations would be exceptions to this rule. In addition, a female could be affected if she were the offspring of an affected male and a carrier female and inherited a deletion or mutation of the STS gene on both X chromosomes. Genetic counseling issues Since the majority of cases appear to occur through transmission of an STS deletion from a carrier mother, enzyme testing or DNA testing should be performed in the mother of any newly diagnosed simplex case (i.e. the first case in a family). In the case of an extended family with many affected individuals, carrier status can often be assigned based on pedigree analysis. Males with XLI will transmit the X chromosome harboring the STS deletion or mutation to each of his female offspring, who will therefore be an obligate carrier. However, all male offspring will be unaffected, since they receive their father's Y chromosome. Female carriers of an STS deletion or mutation have a 50% chance with each pregnancy of transmitting it to an offspring. Thus, each male offspring has a 50% chance of being affected by XLI, while each female offspring has a 50% chance of being a carrier for this condition. Any individual that inherits the mother's normal copy of the STS gene will be unaffected and will have an extremely low chance of having a child affected with this condition. Due to random segregation of the chromosomes during gametogenesis, each pregnancy will be