rhTCII supports the growth of human and mouse cells. Cells were seeded at 2,000 cells/well (in 0.1-mL volumes), cultured in RPMI lacking Cbl where the folic acid was replaced with 1 μmol/L 5-methyltetrahydrofolic acid and 1 μmol/L homocysteine (medium II), and then supplemented with 10% QUSO-treated FCS. Serial dilutions of recombinant TCII were added to cultures. Viability of murine BW5147 cells (A) and human K562 cells (B) was assessed after 5 days using the MTT reduction assay. Results are expressed as the mean ± SEM of six replicate data points. Background represents cell viability in the absence of holo-TCII (•).