Pseudodidymosis aplasticosebacea: phenotype expansion of a mosaic RASopathy 25 years after its identification.

BACKGROUND: The co-occurrence of aplasia cutis congenita (ACC) and nevus sebaceus (NS) was originally framed by Happle and König as "didymosis aplasticosebacea" (now correctly renamed "pseudodidymosis aplasticosebacea" [PAS]). SUMMARY: A search of the worldwide literature up to April 2026 was carried out on the topic. In addition to the already recognized cases, this review yielded the retrospective identification of an additional 25, all diagnosed otherwise, bringing the total to at least 39. Among the cutaneous and extracutaneous associations of pseudodidymosis aplasticosebacea, the latter being reported in all but five cases, ocular and central nervous system abnormalities appeared to be the most prominent ones. A mosaic KRAS mutation was identified in eight individuals. KEY MESSAGES: PAS represents a nonrandom association of lesions, but is still largely unrecognized in the medical-scientific community. Ocular abnormalities, particularly dermoids and colobomas, are by far the most frequent extracutaneous manifestations encountered in PAS. Other peculiar anomalies appear to occur relatively frequently, namely central nervous system lipomas and lymphatic malformations. It is conceivable to definitively include PAS within the realm of mosaic RASopathies.