CS1 down-regulation induces earlier apoptosis after serum deprivation. (A) Pools of CS1null OPM2 and cntOPM2 cells in quadruplicate were incubated with serum-free medium for the indicated periods followed by individual caspase activity assays. ■ represents CS1null OPM2 cells; □, cntOPM2 cells. (B) Cell lysates were prepared from indicated cells after indicated time periods and subjected to immunoblotting using anti-PARP Ab. (C) Cell viability was assayed by MTT at indicated time periods. ■ represents CS1null OPM2 cells; □, cntOPM2 cells. (D) mRNA transcripts (top panel) and protein levels (bottom panel) of proapoptotic molecules BNIP3 and BIK differed in CS1null OPM2 versus control OPM2 cells.