Administration of 5 and 10 μM quercetin-3-methyl ether to wild-type and lapatinib-resistant SK-BR-3 breast cancer cells for 16 and 48 h induced significant accumulation of cells in the G2/M phase, which was correlated with increased levels of cyclin B1/p-cyclin B1 (Ser 147), cell division cycle 225 proteins (Cdc225): Cdc225/p-Cdc225 (Ser216) and check point kinase (Chk) Chk1/p-Chk1 (Ser345) in sensitive SK-BR-3 cells, but not in resistant ones [100]. This evidence concerns the gene CHEK1 and breast cancer.