The presence of the antigen indicates active melanosome formation and thus melanocytic differentiation. It is also expressed in normal fetal melanocytes, but not in normal resting adult melanocytes, regardless of the degree of pigmentation. Upon activation, adult melanocytes can re-express the HMB45-define antigen (as it is expressed in fetal melanocytes). Such melanocytes are activated by variety of stimuli. For example, HMB45-positive cells have been detected in tissue overlying or adjacent to the granulation tissue, hemangiomas, vessel-rich tumor stroma, and basal cell carcinoma. Hair follicle stain occasionally due to associated stimulated melanocytes. Positive HMB45 staining has not been observed with melanocyte in lentigines or overlying fibroblastic proliferation such as keloids, dermatofibromas, and old fibrotic hemangiomas. Non-melanocytic normal tissues do not react with HMB45 antibody.