Chk1: a double agent in cell cycle checkpoints.

Two cell cycle surveillance systems--the DNA damage checkpoint and the spindle checkpoint--guard against genomic instability. The protein kinase Chk1 is a well-established signal transducer in the DNA damage checkpoint. In this issue of Developmental Cell, Zachos et al.(2007) present evidence to indicate that Chk1 also plays a critical role in the spindle checkpoint, suggesting an interplay between the DNA damage and spindle checkpoints.