Objective:To explore the relationship between YKL-40 and the proliferation of breast cancer and its mechanism.Methods:The expression of YKL-40 in breast cancer MCF-7 cells was detected by immunofluorescence assay.Fluorescence microscope was used to observe the conversion efficiency,and Real-time PCR was used to screen the most effective YKL-40 siRNA.Expression levels of PI3K,P-PI3K,AKT,P-AKT of the PI3K/AKT pathway associated proteins was test by Western blot.At the same time,MTT and flow cytometry were validated by YKL-40 siRNA treatment of human breast cancer MCF-7 ceils,the differences of 24h,48h and 72h groups of cell proliferation ability and cell cycle.Result:MCF-7 cell express YKL-40 protein,mainly located in the cytoplasm.Real-time PCR show that siRNA01,siRNA02,siRNA03 compared with NC group YKL-40 gene silencing effect is remarkable.Among of them the strongest silencing effect is siRNA02 (P＜0.01),Western blot show the experimental group than the control group,Total PI3K and AKT remain unchanged while P-PI3K,P-AKT expression decreased (P＜0.05).In the experimental group,the number of G1 cells in the control group was increased (P＜0.01),while the S phase cells decreased (P＜0.01).MTT results showed that the experimental group compared with the control group,the proliferation ability is decreased(P＜0.01).Conclusions This study suggests that YKL-40 can be used as the upstream regulatory factor of PI3K/AKT signaling pathway and affect the process of cell cycle in breast cancer,and then regulate the proliferation of breast cancer,YKL-40 may be a crucial target for the treatment of breast cancer.