A leucine to proline missense mutation, based on a human mutation associated with Emery-Dreifuss muscular distrophy, was introduced into exon 9 at codon 530 by homologous recombination. A single loxP site remained in intron 9 after a neo selection cassette was removed via in vivo cre mediated recombination. Splicing defects surrounding exon 9 were identified by RT-PCR and sequence analyses and putatively resulted in transcript instability. Consistent with reduced levels of lamin A and C transcripts, protein levels were diminished in MEFs derived from mutant mice.