ENU (N-ethyl-N-nitrosourea) mutagenesis is a widely accepted and proven method to introduce random point mutations in the genome. Because there are no targeted knockout strategies available for zebrafish so far, random mutagenesis is currently the preferred method in both forward and reverse genetic approaches. To obtain high-density mutagenized zebrafish, six consecutive ENU treatments are applied at weekly intervals to adult male zebrafish by bathing them in ENU solution. With this procedure an average germ line mutation load of one mutation every 1.0 x 10(5)-1.5 x 10(5) basepairs is reached routinely in our lab.