A tenosynovial giant cell tumor often grows in large joints and although it does not metastasize it can cause debilitating arthritis despite surgery. Recent work from our group identified a specific change in the CSF1 gene that causes this disease. The same CSF1 pathway is also activated in cases of leiomyosarcoma, another tumor arising in the limbs, and those patients usually progress and die. New drugs are available that block this pathway. To test them properly, we need to use a special implantation procedure in experimental mice that will allow the tumors to produce CSF1 and recruit mouse cells to help them grow. Having developed this technique, we will recruit patients having surgery for these tumors to then implement small bits of their tumor in mice. These mice will then be treated with the new anti-CSF1 drugs. We will then excise the tumors and use microscopy and special tests to assess if tumor growth is blocked. If successful, this work could lead to clinical trials in patients.