QUSO treatment of FCS permits measurement of Cbl-dependent growth of mouse and human cells. Cells were seeded at 5,000 cells/well (in 0.1-mL volumes) and cultured in RPMI lacking Cbl and folic acid (medium I), supplemented with 10% FCS (○) or 10% QUSO-treated FCS (•). Serial dilutions of CN-Cbl were added to cultures and viability of murine BW5147 cells (A) and human K562 cells (B) was assessed, after 7 days in culture, using the MTT reduction assay. Results are expressed as the mean ± SEM of three replicate data points. Background represents cell viability in the absence of CN-Cbl for FCS (○) and QUSO-treated FCS (•).