First described by Jamieson  in 1873, and coined by Pringle in 1899, steatocystoma multiplex (SM) is an uncommon disorder of the pilosebaceous unit characterized by the development of numerous sebum-containing dermal cysts. Although steatocystoma multiplex has historically been described as an autosomal dominant inherited disorder, most presenting cases are sporadic. 
Steatocystoma simplex is the sporadic solitary tumor counterpart to steatocystoma multiplex.
Steatocystoma multiplex occurs as either a sporadic or autosomal dominant inherited condition characterized by benign sebaceous gland tumors. Lesions consist of a nevoid formation of abortive hair follicles at the site where sebaceous glands attach. Electron microscopy studies demonstrate cyst wall cells undergoing trichilemmal keratinization similar to that of the isthmus portion of the outer hair sheath. The relationship of steatocystoma multiplex to the development of sebaceous glands and common presentation at puberty suggest a hormonal trigger for lesion growth.
In the familial form of steatocystoma multiplex, mutations are localized to the keratin 17 (K17) gene in areas identical to mutations found in patients with pachyonychia congenita type 2 (PC-2). Pachyonychia congenita type 2, an autosomal dominant inherited disorder, is characterized by hypertrophic nail dystrophy, focal keratoderma, multiple pilosebaceous cysts, and a variety of conditions associated with ectodermal dysplasia. Keratin 17 is expressed in several epithelial structures, most notably in sebaceous glands, the outer root sheath of hair follicles, and the nail bed; its expression correlates well to the clinical phenotypic expression of both steatocystoma multiplex and pachyonychia congenita type 2. To date, 14 mutations have been described in patients with either steatocystoma multiplex or pachyonychia congenita type 2, all of which are localized to the helix initiation domain (1A domain) of the K17 gene. 
Some authors propose that steatocystoma multiplex is simply a variant of pachyonychia congenita type 2 because they both share the same underlying etiology. Sporadic forms of steatocystoma multiplex have not been shown to be associated with K17 mutations. In previous reports, specific mutations were attributed to early-onset cyst formation in pachyonychia congenita type 2 and steatocystoma multiplex; however, more recent reports suggest that the age of onset is multifactorial. 
Steatocystoma multiplex is associated with eruptive vellus hair cysts (EVHCs). Both diseases share overlapping clinical features, including age of onset, location, appearance of lesions, and mode of inheritance. Reports of hybrid lesions showing histological features of both steatocystoma multiplex and eruptive vellus hair cysts exist. [4, 5] Given these similarities, some postulate that steatocystoma multiplex and eruptive vellus hair cysts are, in fact, variants of the same disease.  However, major differences in keratin expression patterns between steatocystoma multiplex and eruptive vellus hair cysts have been elucidated, leading others to believe that they are 2 distinct disease entities.  In steatocystoma multiplex associated with eruptive vellus hair cyst, no K17 mutation has been found.
Steatocystoma multiplex is considered rare; the true incidence is unknown.
No racial predilection has been found.
Both sexes are equally affected.
In the classic presentation, cysts manifest during adolescence and early adulthood, with average age of onset of 26 years.  Cases of steatocystoma multiplex presenting at birth have been reported,  and sporadic forms of steatocystoma multiplex with presentation as late as 78 years have been described.  Once present, steatocystoma multiplex is a lifelong condition.
Steatocystoma multiplex is a benign disorder. In some patients, it may have psychosocial implications resulting from the disfigurement due to widespread lesions or from scarring seen in the inflammatory variant, steatocystoma suppurativa. The prognosis for patients with steatocystoma multiplex is excellent. No reports describe malignant transformation within these benign adnexal tumors.