The four children in this study with MPXPS due to compound heterozygous mutations in the MICU1 gene showed phenotypic differences compared to previously reported MPXPS cases, indicating a positive correlation between MICU1 loss of function and the severity of the phenotype, demonstrating a clear genotype–phenotype correlation. The gene discussed is MICU1; the disease is proximal myopathy with extrapyramidal signs.