Gene abnormalities
Until recently, nodular fasciitis was considered a reaction to trauma or some other unidentified insult at the site where the tumor subsequently developed. However, recent findings indicate that up to 92% of NF tumors involve the self-limiting growth of a clone (i.e. a group of identical cells that share a common ancestry} of neoplastic cells that contain a fusion gene. Fusion genes are abnormal genes consisting of parts from two different genes that form as a result of large scale gene mutations such as chromosomal translocations, interstitial deletions, or inversions. The fusion gene found in NF tumor cells consists of a part of the USP6 gene combined with any one of numerous other genes. Its most common partner gene in NF is the MYH9 (i.e. myosin-9) gene. This USP6-MYH9 fusion gene forms as a result of a translocation of part of the USP6 gene located at band 13.2 on the long (or "q") arm of chromosome 17 with part of the MYH9 gene at band 13.2 on the short (or "p") arm of chromosome 22. Other genes that partner with the USP6 gene to form a fusion gene found in NF include the: RRBP1 (i.e.
ulnar nerve, 2 cases) or median nerve, 1 case), hip/lower limb (i.e. sciatic nerve, 1 case), and lumbar spine/pelvis/thigh (i.e. the obturator nerve, 1 case). Individual cases of NF have been reported to occur in the lower female genital tract, bladder, prostate, tongue, and parotid gland. In some cases, NF tumors have  regressed after incisional biopsy, i.e. a biopsy that takes a small part of the diseased tissue.The cranial fasciitis variant of NF occurs in the soft and hard cranial tissues of the outer layers of the skull. Patients with this variant are more commonly males than females and almost exclusively between 3 weeks and 6 years of age. They typically present with a tumor in areas of the head that lay directly over the temporal or parietal bones. Individual cases have been reported to occur in the lower jaw, frontonasal region, anterior nasal spine (i.e. nasal process of the maxilla), and the orbit (i.e. eye socket)-maxilla area. Characteristically, the tumor is rapidly enlarging, non-painful, rarely regressing without treatment, and potentially expanding into the skulls interior.In a review of 50 cases, the intravascular fasciitis variant of NF occurred in individuals aged 6 months to 66 years (median age 27 years) with male and females being equally affected (52%:48%). Individuals with this variant commonly presented with a blood vessel-localized tumor of the head and neck area (34% of cases), lower extremity (32% of cases), upper extremity (20% of cases), or trunk (14% of cases).