Here, we show that a rare X-linked condition characterized by hypomyelinating leukodystrophy and spondylometaphyseal dysplasia (H-SMD) is caused by specific AIFM1 mutations, all located in a small part of the gene comprising about 70 bp around the start of exon 7. The gene discussed is AIFM1; the disease is spondylometaphyseal dysplasia.