Functional analysis of the mammalian genome will be a major task of biologic science in the future. Gene trap is a method designed to provide functional information for novel genes (1 –3 ). It offers tagging of a gene (facilitating subsequent cloning), analysis of the expression pattern by a simple staining technique, and generation of a mutant mouse strain in one experimental approach. According to our experience (4 ), about two thirds of the genes isolated by this method are either completely unknown or published only as expressed sequence tags. The reporter gene reflects the expression pattern of the endogenous gene faithfully in about three quarters of all cases. Overt phenotypic abnormalities can be expected in about half of the mutant mouse lines generated by gene trap. However, note that frequently the trapped allele is not a null allele, but rather a hypomorphic allele. This issue is discussed in ref. 4 and 5 ). This may be beneficial in cases in which preimplantation lethality would ordinarily preclude the analysis of functions of the gene later in development or in adult life. The most serious criticism to date is that there is little possibility to direct mutagenesis by gene trap to a specific organ system, although attempts in that direction have been made.