Frameshift mutations

Mutations that remove or add a number of bases that are not a
multiple of three will result in an alteration of the transcription
and translation reading frames. These mutations result in the
translation of an abnormal protein from the site of the
mutation onwards and almost always result in the generation of
a premature stop codon. In Duchenne muscular dystrophy,
most deletions alter the reading frame, leading to lack of
production of a functional dystrophin protein and a severe
phenotype. In Becker muscular dystrophy, most deletions
maintain the correct reading frame, leading to the production
of an internally truncated dystrophin protein that retains some
function and results in a milder phenotype.