A loxP site was inserted upstream of exon 2. An FRT-flanked puromycin resistance cassette with a 3' loxP site was inserted downstream of exon 2. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination removed exon 2. Immunofluorescence confirmed the absence of lamin A and C in mouse embryonic fibroblasts.