Most protective functions of the skin, including the permeability barrier and the antimicrobial barrier, reside in the stratum corneum (SC) and the researchers surmise that the SC has undergone the most genetic change since the loss of human body hair. Natural selection would have favored mutations that protect this essential barrier; one such protective adaptation is the pigmentation of interfollicular epidermis, because it improves barrier function as compared to non-pigmented skin. In lush rainforests, however, where UV-B radiation and xeric stress were not in excess, light pigmentation would not have been nearly as detrimental. This explains the side-by-side residence of lightly pigmented and darkly pigmented peoples.Population and admixture studies suggest a three-way model for the evolution of human skin color, with dark skin evolving in early hominids in Africa and light skin evolving partly separately at least two times after modern humans had expanded out of Africa.For the most part, the evolution of light skin has followed different genetic paths in Western and Eastern Eurasian populations. Two genes however, KITLG and ASIP, have mutations associated with lighter skin that have high frequencies in Eurasian populations and have estimated origin dates after humans spread out of Africa but before the divergence of the two lineages. Genetics
The understanding of the genetic mechanisms underlying human skin color variation is still incomplete; however, genetic studies have discovered a number of genes that affect human skin color in specific populations, and have shown that this happens independently of other physical features such as eye and hair color.
Koenens tumor (KT), also commonly termed periungual angiofibroma,: 668  is a subtype of the angiofibromas. Angiofibromas are benign papule, nodule, and/or tumor lesions that are separated into various subtypes based primarily on the characteristic locations of their lesions. KTs are angiofibromas that develop in and under the toenails and/or fingernails. KTs were once considered as the same as another subtype of the angiofibromas viz., acral angiofibromas. While the literature may still sometimes regard KTs as acral angiofibromas, acral angiofibromas are characteristically located in areas close to but not in the toenails and fingernails as well as in the soles of the feet and palms of the hands. KTs are here regarded as distinct from acral angiofibromas. KTs most commonly develop in individuals who have the rare genetic disease, tuberous sclerosis (a heritable neurocutaneous disorder) and uncommonly in individuals that do not have this genetic disease. One individual with another rare genetic disease that has similarities to tuberous sclerosis, the Birt-Hogg-Dube syndrome, has also been reported to have typical KT findings.KTs are strictly benign (i.e. do not metastasize) but may be painful, disfiguring, and sometimes large and/or incapacitating lesions. They have often been treated by strictly local surgical resections. However, they do have a high rate of recurrence at the site of resection and therefore have been treated with various other non-invasive local measures in efforts to avoid recurrences; these other methods have also been preferred to treat numerous tumors in individuals, for cosmetic reasons, and/or to relieve tumor-induce incapacitations.