Since direct cytotoxicity of JMN3-003 was low for all cell lines examined, we next tested the effect of the substance on cell cycle progression. Analysis of the DNA content of cells continuously treated with JMN3-003 for 36 hours by flow cytometry revealed accumulation of cells in a single population with 2N DNA content, which closely resembled the profile of a reference cell population exposed to hydroxyurea but markedly differed from the 4N DNA content of nocodazole-treated cells (figure 4A). Nocodazole interferes with microtubule polymerization [25] , resulting in a G 2 /M arrest, whereas hydroxyurea is thought to lead to an arrest in the G 1 /S-phase through depletion of cellular dNTP pools [26, 27] . To further explore the effect of JMN3-003 on cell cycle progression, we monitored the phosphorylation status of the cdc2-cyclin B kinase after exposure of cells to either the compound, hydroxyurea, nocodazole, or alsterpaullone, a nanomolar small molecule inhibitor of cyclin-dependent kinases that reportedly induces a potent G 1 /Sphase cell cycle arrest [28] . Pivotal in regulating the G 2 /M transition, cdc2-cyclin B kinase is inactivated through phosphorylation during the G 2 -phase. Accumulation in its phosphorylated form thus indicates a G 1 arrest. As in hydroxyurea-and alsterpaullone-treated controls, exposure of cells to JMN3-003 resulted in increased steady state levels of phosphorylated cdc2cyclin B kinase, supporting a G 1 -phase arrest (figure 4B).