JNJ-26481585 (40 mg/kg; p.o.; once daily, for 3 days) as a potent HDAC1 inhibitor p21waf1,cip1 ZsGreen tumors in vivo. JNJ-26481585 induces continuous H3 acetylation in tumor tissue in vivo. JNJ-26481585 (10 mg/kg; once daily; i.p.; for 14 days) strongly inhibits the growth of large pre-established HCT116 colon xenografts.