These diseases underly recessive mutations in muscle-specific exon 15 of FXR1. The milder form (proximal, with minicore lesions [MYOPMIL]) is a result of strongly reduced expression of FXR1, while the very severe form (with respiratory insufficiency and bone fractures [MYORIBF]) is due to expression of a mutant protein prone to aggregation [29]. The gene discussed is FXR1; the disease is myopathy, congenital proximal, with minicore lesions.