A major component of cells in Tenosynovial Giant Cell Tumor (TGCT) consists of bystander macrophages responding to CSF1 that is overproduced by a small number of neoplastic cells with a chromosomal translocation involving the CSF1 gene. An autocrine loop was postulated where the neoplastic cells are stimulated through CSF1R expressed on their surface. Here we use single cell RNA sequencing to investigate cellular interactions in TGCT. Overall design: A total of 18,788 single cells from three TGCT and two Giant Cell Tumor of Bone (GCTB) samples underwent singe cell RNAseq. PacBio IsoSeq was performed on the three TGCT cases.