CSF1 and tenosynovial giant cell tumor, diffuse type: Compared with localized TGCT, diffuse TGCT tends to affect a younger population and is more predominant in women than men.2 Development of TGCT has been associated with a clonal neoplastic process, often involving a specific chromosomal translocation, t(1;2) (CSF-1;COL6A3), resulting in the overexpression of colony-stimulating factor 1 and recruitment of CSF1 receptor (CSF1R) macrophages, giant cells, and osteoclasts.3 A subset of tumors exists (2% to 16%) where there seems to be another effector downstream that results in excess CSF1R expression/response without the mutation.5,6