NEK11, isolated as one of the genes whose knockdown overrides the G2/M checkpoint and shown to also participate in the S phase checkpoint, phosphorylates CDC25A at serine residues S79, S82, and S88 (Melixetian et al. 2009). S82 and S88 belong to the DSG phosphodegron motif required for SCF-BTrCP binding and ubiquitination of CDC25A (Busino et al. 2003, Jin et al. 2003).

NEK11 catalytic activity is stimulated by CHEK1-mediated phosphorylation of NEK11 at S273 (Melixetian et al. 2009). NEK11-mediated phosphorylation of CDC25A is stimulated by a priming phosphorylation of CDC25A at S76 (Melixetian et al. 2009). CDC25A is known to be phosphorylated at S76 by CHEK1 (Goloudina et al. 2003, Hassepass et al. 2003, Jin et al. 2008), suggesting that CHEK1 both activates NEK11 in response to DNA damage and primes CDC25A for NEK11-mediated phosphorylation (Melixetian et al. 2009, reviewed in Sorensen et al. 2010). CHEK1-phosphorylated CDC25A is therefore annotated as a NEK11 substrate. S76 of CDC25A can also be phosphorylated by p38 MAPK (Goloudina et al. 2003) and GSK3B (Kang et al. 2008), so p38 MAPK- and GSK3B-phosphorylated CDC25A proteins are shown as candidate substrates of NEK11.

NEK11-depleted cells fail to degrade CDC25A in response to ionizing radiation (IR) (Melixetian et al. 2009). NEK11 depletion does not affect accumulation of CDC25A in G0 and G1, but leads to a dramatic increase in CDC25A protein levels in S and G2, without a significant change in CDC25A transcript levels (Melixetian et al. 2009).