The Pom1 gradient model for length sensing in fission yeast. (a) An early G2 cell. Pom1 protein emanating from the cell tips (purple) inhibits Cdr1/Cdr2 in the cortical nodes (light blue circles). Wee1 is therefore active and carries out inhibitory phosphorylation of Cdc2. (b) A late G2 cell. Pom1 concentration at the medial site is decreased. At a critical threshold, Cdr1/Cdr2 are no longer inhibited (dark blue circles) and so the Wee1 inhibition of Cdc2 is lifted. The active CDK drives mitotic entry.