Our study aimed to introduce a new perspective on genotype–phenotype correlations and reconsider their predictive value in the XLMTM disease course based on data obtained from databases published over more than 20 years, supported by statistical processing, as well as expansion of the MTM1 mutational spectrum by three novel disease-causing variants identified in our patients. The gene discussed is MTM1; the disease is X-linked myotubular myopathy.