Cell proliferation was estimated with immunostaining for Ki-67 antigen, which is expressed in the nuclei of actively dividing cells but absent at G 0 (23). Ki-67 was recognized with a rat monoclonal antibody to mouse Ki-67 antigen (DAKO) followed by detection with the ABC elite kit using 3,3-diaminobenzidine (DAB) as a substrate (Vector Labs, Burlingame, CA).