Mycosis fungoides (MF) is the most common type of primary cutaneous T cell lymphoma. Many clinicopathological variants of MF have been described in the literature, though only a few presented in a segmental pattern. There are several unique patterns of distribution of skin diseases, one of which is the Blaschko Lines. Congenital skin diseases develop in a Blaschkoid pattern due to mosaicism. In contrast, according to Happle, the development of acquired skin diseases in a similar pattern is explained by superimposed segmental manifestation - a process which involves mosaicism overlapping a preexisting congenital mutation. The theories by which previous case reports explained the segmental appearance of MF did not cover the molecular basis for their development. We report a case of a patient who presented with MF in a unique segmental distribution consistent with the Blaschko lines. The patient was found to have an acquired mosaic mutation in GNAS gene exclusively in the involved skin which represents a superimposed segmental manifestation according to Happle's theory. This case demonstrates the hidden potential of these rare cases which allows a better understanding of the pathogenesis by which acquired diseases develop. This is a basis for further research that could help identify new therapeutic targets for MF and other diseases that share its genetic etiology.