Sex-linked diseases are caused by mutations in genes carried on the X chromosome. X-linked diseases with a recessive pattern of inheritance are the most common. The defective gene on the X chromosome tends to have little effect on heterozygote females because there is a second normal copy of the gene on the other X chromosome. X-linked diseases with a dominant pattern of inheritance are less common and can be lethal to males in utero. The identification of female embryos using fluorescence in situ hybridization (FISH) is a standard technique in most preimplantation genetic diagnosis (PGD) centers. The most common diseases tested by centers contributing cycle data to the European Society for Human Reproduction and Embryology (ESHRE) PGD Consortium using a specific diagnosis were Duchenne muscular dystrophy, hemophilia A, and Fragile X. X-linked disease provides special ethical, counseling, and decision-making challenges beyond those posed by autosomal monogenic defects and chromosome rearrangements.