Conditional insertional mutagenesis strategies. In each case, a vector-integrated locus is shown. Sequences required for Cre-mediated recombination are designated 'LoxP' and shown as a triangle; similarly, sequences required for FLP-mediated recombination are designated 'FRT' and shown as a diamond. These recombinase-requiring sequences are complex components and have been greatly simplified in this diagram. Recombinase sequences that have been converted to an inactive form after inversion are shown hatched. Selection and modification after integration is conducted in mouse embryonic stem (ES) cells in vitro, resulting in a nonmutagenic insertional allele. The locus is shut off in mice in vivo after Cre-mediated inversion and resolution. See text for details. E, exons.