A severe deficiency of spermatogenesis. Chromosome Y deletions are a frequent genetic cause of male infertility. The mode of transmission follows a Y-linked pattern, with incomplete penetrance, but as deletions are often associated with infertility, they generally occur de novo. Molecular diagnosis is made by PCR amplification of STS type sequences (sequence-tagged sites) from the AZFa, b, and c regions. All chromosome Y deletions do not necessarily lead to infertility: firstly, some deletions (especially some partial deletions) do not result in spermatogenesis defects; secondly, among men with severe oligospermia, some can father children without infertility treatment. Finally, when mature spermatozoa are found in the sperm or in the testicles, the infertility problem can be solved with medically assisted procreation techniques. However, there is a risk of transmitting the microdeletion to every male infant.