“…The aim is to inactivate the gene by inserting a piece of foreign DNA into its normal sequence. Gene trapping, that is untargeted insertional mutation based upon random incorporation of DNA sequences throughout the genome, is useful for locating previously unidentified genes [10,11], Poten tially useful mutants arc initially identified by behavioural defects such as loss of hearing or balance. Several important genes have been identified this way [6,[12][13][14], and many more arc likely to follow.…”