Monilethrix is a rare inherited defect of the hair shaft resulting in hair fragility and dystrophic alopecia. In contrast to recent reports mapping monilethrix to the type II epithelial and trichocyte keratin gene cluster on 12q13, we strongly excluded these candidate genes in another family with autosomal dominant monilethrix. Moreover, there was no evidence for linkage of the disease to the keratin gene cluster on chromosome 17q12-q21, thus excluding defects in all known trichocyte and epithelial keratins as the cause of monilethrix in this family. Likewise, several other genes known to play an important role in hair shaft formation (trichohyalin and involucrin, ultra-high sulfur matrix proteins, and transglutaminases 1, 2, and 3) did not provide any evidence for linkage. Our results indicate genetic heterogeneity in monilethrix and suggest that aberrations in at least one other gene result in a similar phenotype.