Keratoacanthoma is a proliferation of the infundibular portion of the hair follicle. They are clonal and can harbor numerous chromosomal abnormalities, including trisomy 7, gains of 1p, 8q, 9q and deletions of 3p, 9p, 19p, and 19q. Translocations involving chromosomes 2 and 8 have also been observed. Most cases are related to excessive exposure to UVB. Patients with xeroderma pigmentosum or chronic immunosuppression are at increased risk. Some cases are associated with inflammatory dermatoses (lichen planus, vitiligo, discoid lupus, atopic eczema, and psoriasis) or scars (previous trauma, radiation, skin grafting).