The acquisition of chemoresistance in the course of tumor progression includes activation of membrane ABC transporters, detoxification enzymes, cell cycle deceleration and activation of specific signaling pathways such as Akt/mTOR, MAPK, NF-κB. Adaptor proteins play an essential role in the assembly of supramolecular signaling complexes, maintaining and directing the intracellular signaling. One of such proteins, called Ruk/CIN85, is strongly associated with malignant transformation and metastasis. In present study we investigated the Ruk/CIN85 effect of up/down-regulation on the transforming potential and doxorubicin resistance of highly aggressive mouse breast adenocarcinoma 4T1 cells. It was demonstrated that 4T1 cells overexpressing Ruk/CIN85 possessed increased resistance to doxorubicin (in the range of concentrations 0.1–10.0 µM) while knockdown cells were the most sensitive. Also, high levels of Ruk/CIN85 in 4T1 cells positively correlated with their ability to form colonies in semi-solid agar. Ruk/CIN85-overexpressing cells formed four times more colonies in comparison with Ruk/CIN85 nockdown cells, the growth of which revealed higher resistance to doxorubicin action.