Recombinant apo-TCII reduces the concentration of Cbl required to support in vitro cell growth. 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 CN-Cbl were added to cultures. After 5 days in culture, viability of BW5147 cells (A) and K562 cells (B) was assessed using the MTT reduction assay. (○), No apo-TCII; (•), 25 ng/mL apo-TCII. Results are expressed as the mean ± SEM of 3 replicate data points. Background represents cell viability in the absence of Cbl and apo-TCII (○) or absence of Cbl and presence of 25 ng/mL apo-TCII (•).