Trichoepithelioma is a benign adnexal neoplasm. According to some authors, trichoepithelioma may be a superficial form of trichoblastoma. The gene involved in the familial form of trichoepithelioma is located on band 9p21.  Other cases are associated with Brooke-Spiegler syndrome caused by mutations of the cylindromatosis oncogene (CYLD), which maps to 16q12-q13. [2, 3, 4] A 2006 study has suggested that abnormalities in this gene may result in one of 3 syndromes: Brooke-Spiegler syndrome, familial cylindromatosis, and multiple familial trichoepithelioma.  Furthermore, some cases of patients with multiple trichoepitheliomas appear to be sporadic and not related with familial incidence. 
A 2009 study reports a novel missense mutation in the CYLD gene, heterozygous nucleotide G-->A transition at position 2,317 in exon 17, in a Chinese family with multiple familial trichoepithelioma.  Additionally, a novel splicing mutation in the CYLD gene (IVS12 + 1 G-->A) has been reported in a Taiwanese family with multiple familial trichoepithelioma. 
The gene associated with the familial type of trichoepithelioma links to the short arm of chromosome 9. Because several tumor suppressor genes (ie, p16, p15, and the gene for the basal cell nevus syndrome) are in this region, the gene for the development of familial trichoepithelioma also encodes for a tumor suppressor. If altered, cellular proliferation may be up-regulated because of a poorly functioning or absent tumor suppressor. Studies have indicated that CYLD encodes a deubiquitinating enzyme that negatively regulates the nuclear factor (NF)–kappaB and c-Jun N-terminal kinase (JNK) pathways.  Due to the presence of significant numbers of Merkel cells within the tumor nest and the detection of a sheath of CD34-positive dendrocytes around the tumor nests, it appears that trichoepithelioma differentiates toward or derives from hair structures, particularly the hair bulge. Rare instances of tumors resembling trichoepithelioma have been reported in animals. 
One dermatopathology laboratory reported 2.14 and 2.75 cases of trichoepithelioma per year (9000 specimens).
Since trichoepithelioma is inherited in an autosomal dominant fashion, males and females receive the gene equally, but because of lessened expressivity and penetrance in men, most patients are women.
Trichoepithelioma typically occurs in young to aging adults; however, the hereditary form may be seen in younger individuals. A single case study has reported a congenital lesion of desmoplastic trichoepithelioma. 
Slow growth is characteristic of trichoepithelioma. Partial removal may result in persistence or recurrence. Although rare, trichoepitheliomas can develop high-grade carcinomas and mixed (epithelial/sarcomatous) tumors. [12, 13] Familial trichoepithelioma has shown an aggressive, recurrent behavior in rare cases.
In cases of multiple trichoepitheliomas, the lesions may cause disfigurement because of involvement of the face. The rare cases of trichoepithelioma described as having aggressive behavior (ie, ulceration, recurrence) are probably follicular tumors within the basal cell nevus syndrome and not trichoepithelioma.
Inform the patient that some degree of scarring will be present after trichoepithelioma treatment.