Comparison of Bcl-2, Mcl-1, Bcl-x_L, and Bax levels in paired leukemia specimens harvested before chemotherapy and at the time of leukemia recurrence. Odd numbers, pretreatment specimens. Even numbers, corresponding samples at recurrence. To permit quantitation, a serial dilution of HL-60 cells was run on each blot as depicted in Fig 1. Lanes 1 and 2 contain a pair displaying no twofold change in protein expression between diagnosis and relapse. Lanes 3-8 contain pairs displaying more than twofold increases in Mcl-1 expression at relapse. In two cases (lanes 3 through 6) there was no change in Bcl-2 expression; but in one case Bcl-2 expression decreased (lanes 7 and 8).