Loss-of-function mutations in MICU1, a regulator of the inner mitochondrial complex MCU, responsible for regulating mitochondrial CA2+ uptake and preserving normal mitochondrial CA2+ concentration are reported to cause a childhood-onset disease with raised CK, relatively static proximal myopathy, variable CNS involvement, and distinctive biopsy features including preserved fibre typing, mild central nucleation, mini-cores and clustered regeneration. Here, CA2 is linked to myopathy.