LAMA2 and Menkes disease: To circumvent these extrinsic factors and to test whether loss of Lama2 expression in MuSCs directly impairs proliferation, we generated mice that carry loxP sites in the introns flanking exon 3 of Lama2. Deletion of exon 3—which is commonly observed in LAMA2 MD patients38—results in frameshift mutations and the introduction of stop codons from exon 4 onwards39.