RO4929097 is an orally bioavailable, small-molecule gamma secretase (GS) inhibitor with an IC50 of 4 nM. It binds to GS and blocks activation of Notch receptors, which may inhibit tumor cell proliferation. RO4929097 inhibits Notch processing in tumor cells as measured by the reduction of intracellular Notch expression by Western blot. This leads to reduced expression of the Notch transcriptional target gene Hes 1. RO4929097 does not block tumor cell proliferation or induce apoptosis but instead produces a less transformed, flattened, slower-growing phenotype.