Aggressive fibromatosis (desmoid tumor) is a monoclonal disorder.

Aggressive fibromatosis (also called deep fibromatosis or desmoid tumor) is a proliferation of cytologically benign-appearing fibrocytes, often resulting in significant functional loss. The nature of the lesion is controversial: some evidence suggests that it is a reactive process, whereas other evidence supports a neoplastic etiology. The pattern of X chromosome inactivation, using a technique based on polymerase chain reaction (PCR) amplification of a hypervariable CAG repeat region flanking Hhal restriction sites of the human androgen receptor gene, was determined in four cases in which cryopreserved tumor and adjacent normal tissue were available. All four tumors demonstrated a monoclonal pattern, while the adjacent normal tissues demonstrated a polyclonal pattern. This demonstrates that aggressive fibromatosis is proliferation of cells derived from a single clone with a growth advantage, and thus is likely a neoplastic process.