Fluorescent in situ hybridisation (FISH) is a diagnostic technique that uses fluorescent probes to analyse part of the genetic material of spermatozoa. In the usual analysis, a maximum of 9 of the 23 chromosomes that make up sperm DNA are evaluated. These chromosomes commonly include 13, 15, 16, 17, 18, 21, 22, as well as the X and Y sex chromosomes.