2.2. Cell culture and generation of drug‐tolerant cells

Human pro‐B MLL leukemia cell lines RS4;11 and SEM ³⁴ were purchased from DSMZ. Cells were cultured in Gibco RPMI‐1640 containing 10% FBS at 37°C with 5% CO₂ and were maintained between a density of 5 × 10⁵ cells/mL and 2 × 10⁶ cells/mL. The generation of induced drug‐tolerant cells has been described previously. ²⁰ Briefly, naïve cells were exposed to a sublethal dose of bortezomib (5 nmol/L) for 2 wk, replenishing the inhibitor every 3 d. The remaining cells after the treatment were considered as “Tolerant” cells and were collected for analysis. “Reverted” cells were generated from bortezomib‐tolerant cells by culturing without bortezomib for a minimum of 4 wk. All cultured cells were tested for mycoplasma contamination before use.