Possible or probable XLMTM in dogs can be predicted for litters containing exclusively affected males with a skeletal myopathy of early onset and rapidly fatal course, together with characteristic skeletal muscle pathological findings, and should lead to diagnostic sequencing of the entire MTM1 gene as this condition may be due to multiple independent mutational events regardless of breed or geographic origin. This evidence concerns the gene MTM1 and X-linked myotubular myopathy.