CDKN1B and breast cancer: In the next experiment, we used various chemically synthesized retinoic acids to investigate whether the degree of up-regulation of the -1797 p27-luciferase reporter activity (p27-Kpn I) in human breast cancer cells in vitro correlates with the degree of inhibition of methylnitrourea (MNU)-induced rat mammary adenocarcinoma in vivo.