has neoplastic cells with an extra, abnormally-shaped small supernumerary marker chromosome(s). These chromosomes consist of a small number of genes copied from the q arm of chromosome 12 at sites 13-15 (notated 12q13–q15) containing and thereby amplifying expression of the CDK4 and MDM2 genes and these genes CD4K glycoprotein and MDM2 protein products. ATL/WDL neoplastic cells generally express pRB. WDL is differentiated from MFB by its histology and detecting these abnormal chromosomes. Desmoid fibromatosis, also termed desmoid-type fibromatosis, is a slow growing, rarely multifocal tumor that does not metastasize but is locally aggressive. About 10% of cases develop in individuals with the inherited disease, familial adenomatous polyposis (FAP). FAP is due to inherited mutations in the APC gene responsible for making adenomatous polyposis coli (APC) protein. The remaining 90% of cases with this tumor have mutations in the CTNNB1 gene responsible for making beta-catenin protein. The two mutations cause respective losses in APC protein or excessive accumulations of APC protein. Dermatoid fibromatosis tumors consist of uniform myofibroblasts in a background of abundant collagenous stroma and vascular tissue. The disease differs from MFB in its histopathology, absence of APC protein, excessive beta-catenin protein, and absence of desmin protein. Solitary fibrous tumor (SFT) is a rarely metastasizing but locally aggressive tumor. It originates in the lung plerua or, rarely, abdominal cavity and a wide range of other areas. SFT consists of CD34-positive mesenchymal spindle cells in a highly fibrous and vascular matrix. Rarely, this tumor contains fat cells and thereby can be confused with MFB. It is distinguished from MFB by containing tumor cells which overexpress STAT6 protein as determined by immunohistochemical analysis. The expressed STAT6 in SFT is actually a fusion protein formed by a mutation in which part of the NAB2 gene fuses with the STAT6 gene to produce a (NAB2)-STAT6 chimeric gene whose fusion protein product is detected by specific immunohistochemistry assays for STAT6 protein. Fibromas can have the same prognoses and morphological as well as 13q14-deleted/pRB-negative neoblastic cells as MFB but differ from MFB in that their neoplastic cells do not express desmin. These fibromas are benign and could justifiably be categorizes as MFB variants. Pseudoangiomatous stromal hyperplasia (also termed pseudoangiomatous stromal proliferation) is a benign breast tumor that occurs most frequently in pre-menopausal women and men with gynecomastia. It consists of interconnecting, angulated, slit-like spaces lined by slender spindle cells and surrounded by dense collagenous stroma. This