A method of inducing transposition in a transgenic embryo, sperm and egg is described, comprising the steps of (a) generating a first adult transgenic organism comprising within its genome one or more copies of a transposon; (b) generating a second adult transgenic organism comprising within its genome one or more copies of a gene encoding a transposase cognate for the transposon and/or a sequence capable of regulating expression of the gene encoding the transposase; (c) crossing the first adult transgenic organism with the second transgenic adult organism to provide a progeny which comprises, in the genome of one or more of its cells, both (i) one or more copies of the transposon and (ii) a gene encoding a transposase cognate for the transposon, wherein the gene encoding the transposase is under the control of one or more inducible regulatory sequences which permit expression of the transposase, and (d) expressing the gene encoding the transposase in the embryo, sperm or egg to cause mobilization of the transposon within a portion of the tissues or cells of the progeny. Using the method, mobilization of a transposon can advantageously be induced at predetermined stages of development of an embryo, sperm or egg and the mutated gene of a single cell may be replicated in subsequent cell divisions, resulting in groups of cells which are essentially homogenous for the transposed gene.