Café-au-lait macule (CALM) is a theoretical consideration; however, CALM is unlikely given the absence of syndromic features, the adult-onset acral site, the mottled clinical pigmentation, and a histologic pattern dominated by keratinocyte macromelanosomes with normal melanocyte density and architecture (Melan-A/SOX10), rather than the uniform basal hypermelanosis typical of CALM. The gene discussed is SOX10; the disease is Cafe-au-lait spot.