angiofibroma variant of MFB contain spindle cells in all cases, fat cells in ~50& of cases, mast cells, peri-vascular infiltrates of lymphocytes, pleomorphic cells in some cases, and cells with some features of the malignant sarcoma cells in sarcoma tumors in rare cases. (The presence of these sarcoma-like cells does not seem to impact the prognosis of this variant.) All of these cells are in edematous-to-fibrous stromatous tissue. The neoplastic cells in mammary and extramammary MFB have a small deletion in the long (i.e. "q") arm at site 14 in one of their two chromosome 13s (deletion notated as 13q14). This leads to a loss in expression of pRb, a protein made by the Rb gene located at the deletion site. The presence of pRb is commonly determined by immunohistochemical detection of it in the nucleus of the neoplastic cells; this method finds no pRb in up to 92% of all MFB cases. The presence of two marker proteins in MFP neoplastic cells, also detected by immunohistochemical analyses, support the diagnosis of MFB. These proteins are CD34, a cell surface glycoprotein present in up to 89% of MFB cases, and desmin, a protein located within cells present in up to 91% of MFB cases. Neoplastic cells in the spindle cell lipoma tentative MFB variant, similar to mammary and non-mammary MFB, express CD34 and desmin and, in 70% of cases, have the 13q14 defect and therefore do not express pRb. The cellular angiofibroma tentative variant's neoplastic cells also lack pRb in most cases and express CD34 but express desmin in only a small percentage of cases. Neoplastic cells in mammary and extramammary MMB commonly express the estrogen, progesterone and androgen receptors. Incomplete studies on the cellular angiofibroma tentative variant find the presence of estrogen and progesterone receptors on the neoplastic cells in at least some cases while studies in the spindle cell lipoma variant's neoplastic cells find only a minority (<20%) of cases have estrogen receptor-positive neoplastic cells. Progesterone receptor-positive neoplastic cells occur in only rare cases of this variant. Differential diagnosis MFB can resemble and should be distinguished from several types of tumors most of which carry a more serious prognosis than MFB. Well-differentiated liposarcoma (WDL), also termed atypical lipomatous tumor (ATL) or ATL/WDL, is an extremely rare tumor which unlike MFB is composed of mature, morphologically homogenous fat and stromal cells that frequently have atypical nuclei. This tumor characteristically