The degree of up-regulation of p27 expression in human breast cancer cells in vitro linearly correlates with the degree of inhibition of methylnitrosourea (MNU) -induced rat mammary adenocarcinoma in vivo. (a) Various chemically synthesized retinoic acids were used to investigate whether the degree of up-regulation of -1797 p27 gene-luciferase reporter (p27-Kpn I) activity in estrogen receptor (ER)-negative MDA-MB-231 human breast cancer cells in vitro correlates with the degree of inhibition of methylnitrourea (MNU) -induced rat mammary adenocarcinoma in vivo . The cells were exposed to 1 μM each of the retinoic acids for 24 hours. In vitro transfection assays were performed in triplicates and repeated three times. (b) Chemical structure of the retinoic acids used in this experiment . (c) Graphical representation of the results in Figure 2a above.