MLS is an X-linked disease caused by the loss or dysfunction of the XK protein due to loss-of-function mutations in the XK gene on the X chromosome.[1–3] Based on exon sequencing analysis, we found a previously unreported frameshift mutation in exon 2 (c.452delA), resulting in amino acid residue conversion from Gln151 to Arg151. The gene discussed is XK; the disease is X-linked disease.