CCND1 and neoplasm: Regarding the cell cycle, KLF4 can cause either G1 or G2 arrest in different cell types.32, 33, 34 Studies have indicated that KLF4 functions as a tumor suppressor in regulating cell proliferation and apoptosis on the basis of the modulation of downstream genes, such as those of p27,30 p21,35 p57,36 cyclin D1 (ref. 37) and cyclin B1,38 and that KLF4 has potent tumor suppressor-like function.