The p.R50X mutation (c.148A4T nucleotide change, GenBank accession number NM_011224.1) was introduced in exon 1. A loxP site flanked neomycin resistance cassette, under the human phospho-glycerate kinase promoter (LoxP-PGK-NEO), was inserted in intron 1. The neo cassette was removed through subsequent cre-mediated recombination. Glycogen phosphorylase activity was totally abolished in the gastrocnemius muscle of homozygous mice.