MCL cells with the highest ALDH activity have enhanced clonogenic capacity and are more quiescent than bulk cells. (A) ALDH analysis of the human MCL lines Granta 519, Jeko-1, and Rec-1 and a representative clinical specimen by flow cytometry. ALDH⁺ populations are defined by gating for live cells by forward side scatter and in parallel with diethylaminobenzaldehyde (DEAB)–treated controls. Positive gates are determined by DEAB-treated cells and set at 0.01% for DEAB controls. For the patient samples, MCL tumor cells were initially identified by gating for CD19⁺CD5⁺ cells. Values represent percentage within each gate. (B) Colony formation by 1000 ALDH⁺ or ALDH^neg cells. (C) Cell cycle analysis of ALDH⁺ and ALDH^neg Jeko-1 cells. Values represent percentage within each gate. (D) Frequency of ALDH⁺ and ALDH^neg cells in G₀/G₁ (*P < .05; n = 3). SSC indicates side scatter; 7AAD indicates 7-amino-actinomycin D.