Protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) are involved in the regulation of basic cell functions, such as proliferation, differentiation, and death. This review introduces our preclinical studies of the roles of PKC and MAPK signaling in breast cancer cell-cycle regulation. PKC caused G₁ arrest in a breast cancer cell line via a mechanism involving a MAPK–c-jun NH₂-terminal kinase (JNK)–retinoblastoma (Rb) protein signaling pathway. Furthermore, we characterized a novel mechanism by which all-trans retinoic acid (ATRA), antineoplaston anticancer drugs, and catechin inhibited the growth of breast cancer cells via effects on intracellular pathways. ATRA decreased the expression of PKCα, reduced extracellular signal-regulated kinase (ERK) MAPK phosphorylation, and, consequently, caused G₁ arrest. Antineoplastons downregulated PKCα protein expression, resulting in the inhibition of ERK MAPK phosphorylation and Rb phosphorylation, leading to G₁ arrest. Catechin phosphorylated JNK and p38, which, in turn, inhibited the phosphorylation of cdc2, and regulated the expression of cyclin A, cyclin B1, and cdk proteins, thereby causing G₂ arrest. These results suggest that PKC and MAPK are essential for cell-cycle regulation in breast cancer cells. PKC and MAPK signaling thus represent promising targets for the development of novel therapeutic agents for breast cancer.