During mitosis, Promyelocytic leukemia nuclear bodies (PML NBs) change 
dramatically in morphology and composition, but little is known about function 
of PML in mitosis. Here, we show that PML is phosphorylated at T409 (PML p409) 
in a mitosis-specific manner. More importantly, PML p409 contributes to maintain 
the duration of pro-metaphase and regulates spindle checkpoint. Deficient PML 
p409 caused a shortening of pro-metaphase and challenged the 
nocodazole-triggered mitotic arrest. T409A mutation led to a higher frequency of 
misaligned chromosomes on metaphase plate, and subsequently death in late 
mitosis. In addition, inhibition of PML p409 repressed growth of tumor cells, 
suggesting that PML p409 is a potential target for cancer therapy. Collectively, 
our study demonstrated an important phosphorylated site of PML, which 
contributed to explore the role of PML in mitosis.