and then embryos. These arrays can be transmitted to most cells of the resulting animals and through the germ line to their progeny. Although the genes on such arrays are present in high and somewhat variable copy number, they are efficiently expressed and can be useful for many types of investigations, such as transformation rescue in positional cloning and analysis of a cloned gene’s expression patterns using lacZ or GFP reporter-gene constructs. From transmitting lines, more stable integrated lines can be obtained in which the array has inserted randomly into a chromosomal locus, allowing various gene-trapping technologies for identifying loci with tissue-specific expression patterns. Targeted insertion of transgenes by homologous recombination has not yet been achieved. Reverse genetics using targeted gene disruption is therefore difficult but can be accomplished by random transposon insertion (Plasterk 1995) or deletion mutagenesis followed by appropriate screens, or it can be accomplished by RNA-mediated gene interference (RNAi), as discussed next.