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In mouse, the skeletal muscle-specific isoform contains 6 additional exons included after birth and is the predominant isoform in adult skeletal muscle. CRISPR/Cas9 genome editing technology was used to delete exons 9-14 encoding the skeletal muscle-specific isoform, thereby forcing the constitutive expression of the ubiquitous isoform that normally predominates in fetal skeletal muscle. RT-PCR analysis demonstrated the absence of the 6 alternatively spliced exons and the correct splicing of the flanking constitutive exons 8 and 15 that are included in RNA from knockout skeletal muscle tissues. Western blotting confirmed that knockout mice lack protein expression of the skeletal muscle-specific isoform and exclusively express the fetal predominant isoform in adult skeletal (quadriceps, EDL, and soleus) muscle. (J:336773)