IDO1 and synovial sarcoma: In an in vitro study using samples of T cells from 68 SS patients, coculture with mesenchymal stem umbilical cells revealed the suppression of proliferation and activation of these circulating follicular T helper cells, in association with the enhanced expression and enzymatic activity of IDO, as measured by reversal transcription polymerase chain reaction (RT-PCR) and high performance liquid chromatography (HPLC), respectively [249].