BACKGROUND Co-occurrence of aplasia cutis congenita and nevus sebaceus has sometimes been observed. OBJECTIVE We propose a genetic mechanism that may explain this coexistence of two different congenital skin disorders. METHODS AND RESULTS We review 15 cases as reported in the literature and present an additional case. To explain the temporal and spatial proximity of the two skin lesions we advance the concept of twin spotting for which we propose the term didymosis (Greek didymos = twin). In an embryo heterozygous for two different recessive mutations localized on either of a pair of homologous chromosomes, an event of somatic recombination would occur at an early developmental stage, giving rise to two different cell clones homozygous for either mutation. CONCLUSION The concept of didymosis aplasticosebacea is so far hypothetical. Future molecular research may show whether this concept holds true.