Previous reports just demonstrate that AMG900 induces formation of polyploid cells with low level of H3‐pS10 in liposarcoma cells,67 breast cancer cells,18, 19 prostate cancer cells,22 and colon cancer cells.20 AMG900‐treated colon cancer cells entering mitosis abort cell division without proper chromosome congression and alignment.20 Here, we show that AMG900 causes accumulated chromosomal instability by defects in mitotic spindle formation, SAC activation, chromosome segregation, and cytokinesis. The gene discussed is ADCY10; the disease is breast cancer.