Consistent with this, yeast cells lacking function of Yph1p, the yeast pes homolog, underwent cell cycle arrest [10], Pes1 mutant mouse embryos arrested as early as the eight-cell stage [12] and knockdown of pes1 function by RNAi in two breast cancer cell lines resulted in a significant inhibition of cell growth [15]. The gene discussed is PES1; the disease is breast cancer.