In the MAPK module, MEK lies upstream of ERK which is found constitutively activated in a host of human tumors. We already demonstrated the growth inhibitory activity of MEK inhibitors in myeloid cells (JCI 2001, Leukemia 2005). PD0325901 is the latest small-molecule inhibitor of MEK with promising effects at lower concentrations. We tested its activity (0.1–1000 nM) in a broad spectrum of leukemia, melanoma and breast cancer cell lines evaluating changes on cell cycle distribution, apoptosis, protein and gene expression profiles, aiming at defining the molecular signature induced by this MEK inhibitor. Among hematopoietic cell lines, PD0325901 induced a marked growth inhibition in myeloid cells with constitutive ERK activation (IC50=11 and 12 nM for OCI-AML3 and OCI-AML2, respectively). Conversely, relative resistance to PD0325901-mediated growth inhibition (IC50>1μM) was observed in myeloid cell lines without constitutive ERK activation (U937, KG-1) and in lymphoid cell lines (Raji, Jurkat). Among the solid tumor cell lines, the M14 melanoma was markedly sensitive to PD0325901-induced growth inhibition (IC50=24 nM), even with forced Bcl-2 expression (IC50 ranging from 64 to 200 nM in Bcl-2-overexpressing clones). Conversely, the breast cancer cell lines tested (SKBr3, BT474, MDA-MB-231 and ZR75-1) proved relatively resistant (IC50≥1 μM), regardless of the ERK phosphorylation status. In responsive cells (OCI-AML3, OCI-AML2 and M14), PD0325901 inhibited ERK phosphorylation in a dose-dependent manner, the effect was already evident at 15 min. Cell cycle distribution analysis demonstrated a dramatic dose-dependent decrease in the proliferative compartment in OCI-AML3. Subsequently (48–72 h), PD0325901 induced apoptosis in a dose- and time-dependent fashion, as demonstrated by the increase in the percentage of AnnV+ cells from 6.3%±1.1 (DMSO control) to 15.5%±3.9, 31.5%±2.8 and 45.3%±0.14 (10, 100, and 1000 nM PD0325901, respectively). Gene expression profiling was conducted using AffymetrixTM HG-U133A 2.0 GeneChip® in OCI-AML3 cells exposed to DMSO (control) or PD0325901 at 10nM. Using a p<0.05 and a fold changes ≥ 2.0 cut-off, 16 genes were found to be differentially expressed (3 upregulated and 13 downregulated) after 6h of PD0325901 treatment. These effects were even more pronounced after 24h of treatment and, in addition, at this time other genes turned out to be modulated by PD0325901 treatment (a total of 37 were upregulated, 59 downregulated); PD0325901 induced transcriptional changes mostly in genes responsible for cell growth/proliferation, DNA replication and cell signalling. In conclusion, we found that PD0325901, at nanomolar concentrations, displays a promising growth-inhibitory and pro-apoptotic activity in cells with constitutive ERK activation (particularly myeloid leukemias and melanoma). We demonstrated that the expression gene profile of OCI-AML3 is profoundly altered by PD0325901 treatment, particularly reflecting changes in genes involved in the MEK-dependent regulation of cell cycle, as well as new genes potentially useful candidates for further investigation.