Mastocytosis is a blood disorder characterised by abnormal mast cell proliferation and accumulation in diverse organ systems, mainly the skin. Cutaneous mastocytosis (CM) demonstrates a very wide clinical spectrum ranging from isolated cutaneous lesions to the generalized involvement of the skin. Cutaneous mastocytosis include the following types: solitary mastocytoma, diffuse erythrodermic mastocytosis, paucicellular mastocytosis (named also as telangiectasia macularis eruptive perstans [TMEP]), and urticarial pigmentosa (UP). UP is considered the most common form. The majority of patients are children where 75% of cases appear throughout infancy or early childhood and usually settle by puberty. Rarely, adult-onset mastocytosis can cause mast cell leukemia, which is an aggressive type of leukemia associated with high mortality risk.

There are some indications that sporadic non-familial cases of urticaria pigmentosa may be related to mutations in the V-Kit Hardy-Zuckerman 4 Feline Sarcoma Viral Oncogene Homolog (KIT) gene located at 4q12. Mutations in the TET Oncogene Family, Member 2 (TET2) gene are also frequent in systemic mastocytosis and segregate with the D816V mutation in KIT.