Fem-1 Homolog B (FEM1B)

Depletion of FEM1B by small interfering RNA in cancer cells impairs the activation of CHK1 kinase activity and attenuates the induction of CHK1 Ser345 phosphorylation upon replication interference. It is to be noted that, CHK2 Thr68 phosphorylation is not altered by FEM1B downregulation.

FEM1B is able to associate with chromatin, and such association facilitates chromatin loading of the Rad9 protein. Consistently, ATR activity is poorly maintained in FEM1B knockdown cells; and FEM1B-ablated cells are as sensitive to replication block as CHK1-depleted cells. FEM1B, which acts as a bridge linking CHK1 and Rad9, thus facilitating checkpoint signaling induced by replication stress.