CDK9 and neoplasm: Thus, the control of Notch signaling by Cdk5 in the neovascularizing endothelium is proposed as an additional target for tumor treatment although up to now, absolutely selective inhibition specifically of Cdk5 is not possible as the currently available Cdk5 inhibitors interfere with other Cdks like Cdk1, Cdk2, Cdk7, and Cdk9 as well.