TK2 and myopathy: TK2 deficiency manifests predominantly as a progressive myopathy with a broad spectrum of severity ranging from extremely severe and rapidly progressive early-onset forms (with a survival of less than two years) to less severe forms with a childhood, late or very late-onset, and a slower rate of progression, but with an almost invariable respiratory involvement, which appears during disease progression, and shortens life expectancy [3,4,5].