As an anti-microtubule agent, paclitaxel can inhibit proliferation of human endothelial cells with IC50 from 1 pM to 100 pM. Cells began accumulating in G2/M after exposure to paclitaxel. Paclitaxel inhibits the progression of mitotic cells to G1 phase. Paclitaxel-treated cells eventually reform nuclear membranes around individual or clusters of chromosomes, revert to G1 phase cells containing 4C DNA, and enter S phase, resulting in cells with as much as 8C DNA content.