Chromosome analysis of oocytes, sperm and embryos has mainly relied on fluorescent in situ hybridisation (FISH) and karyotyping. FISH studies have been performed on sperm from fertile and infertile men as well as men carrying known chromosomal translocations. Molecular DNA analyses has aided in the identification and treatment of men with Y chromosome deletions. In oocytes FISH and karyotyping have identified non-disjunction of univalents and predivision of chromatids. Analysis of the chromosomes from human embryos has shown that a high proportion of embryos are mosaic or chaotic, in addition to embryos beings uniformly and abnormal. FISH and PCR have also been used clinically for preimplantation genetic diagnosis (PGD). For patients at risk of transmitting a specific genetic or chromosomal abnormality, 1-2 blastomeres are biopsied from embryos and specific genes or chromosomes analysed. Normal embryos are then transferred to the uterus.