Chemical mutagenesis (left). Adult males that have been mutagenized by treatment with ENU are crossed to a wild-type female to create an F1 generation that contains a random set of point mutations in their genome. In a diploid-based screen, members of the F1 are outcrossed to wild type to increase the number of fish carrying specific recessive mutations. The F2 generation is subsequently intercrossed to generate F3 progeny, which can be analyzed phenotypically for recessive defects. One-fourth of the F2 family intercrosses will produce mutant progeny in one-fourth of the F3 embryos. In a haploid screen, eggs obtained from F1 females are fertilized with UV-treated sperm to generate haploid embryos. The haploid clutch derived from a heterozygous female (+/m) will contain 50% mutant and 50% wild-type embryos. Insertional mutagenesis (right). Virus is injected into 1,000- to 2,000-cell–stage embryos. F1 fish carrying more then 3 insertions are subsequently bred. The F2 generation is screened employing the same breeding scheme used for the chemical-based mutagenesis.