Epithelial-mesenchymal transition (EMT) in epithelial cells has been indicated as an important component of neoplastic transformation although, the genetic mechanism involved in this process has not been defined. The aim of this study was to evaluate the expression of different genes related to EMT such as E-cadherin, TGFbeta1, TGFbeta2, h-RAS, TWIST1, SNAIL2, SMAD5, FN1, CEACAM1 and JAG1 using the in vitro-in vivo model of the estrogen induced cell transformation developed in our laboratory. The E2-transformed MCF-10F (E2 70) cells and the tumorigenic cell line C5-A8-T8 (C5-T8) exhibit progressive loss of ductulogenesis as demonstrated by growth in collagen matrix. MCF-10F cells form ductal structures while E2 70 cells form solid spherical masses that in histological sections exhibit a pattern of growth resembling ductal hyperplasia or carcinoma in situ. The tumorigenic cells C5-T8 did not form structures on collagen acquiring an invasive pattern with spindle like features. We have observed a reduction in E-cadherin expression in E2 70 cells and a complete loss in C5-T8 cells. TGFbeta1, TGFbeta2, CEACAM1 and JAG1 were down-regulated in E2 70 and C5-T8 cells. SMAD5 and h-RAS were up-regulated in the tumorigenic C5-T8 cells whereas FN1, Twist1 and Snail2 were up-regulated in C5-T8 and down-regulated in E2 70. We conclude that the loss of expression of TGFbeta1, TGFbeta2, CEACAM1 and JAG1 are related to ductulogenesis and branching and the overexpression of h-RAS with loss of E-cadherin expression and up-modulation of TWIST1, SNAIL2 and SMAD5 expressions are involved in the EMT modulation.