PAK4 and breast cancer: After transient transfection of two independent small interfering RNAs (siRNAs) targeting the human PAK4 gene, Hs 578T breast cancer cells adopted a flatter and larger senescence-associated morphology and exhibited elevated SA-β-gal activity (as measured with the two substrates X-Gal29 and MUG30) that was accompanied by a significant decrease in BrdU-incorporation (Fig. 3a–d).