APOBEC3B belongs to a protein family of cytidine deaminases that can insert mutations in DNA and RNA as a result of their ability to deaminate cytidine to uridine. It has been shown that APOBEC3B-catalysed deamination provides a chronic source of DNA damage in breast cancers. We investigated APOBEC3B expression in four drug resistant breast cancer cell lines (Doxorubicin, Etoposide, Paclitaxel and Docetaxel resistant MCF-7 cell lines) using a novel RNA in situ hybridization technology (RNAscope) and compared expression levels with drug sensitive MCF-7 cell line. After RNAscope staining, slides were scanned and saved as digital images using Aperio scanner and software. Quantitative scoring utilizing the number of punctate dots present within each cell boundary was performed for the parameters including positive cell percentage and signal intensity per positive cell. In Doxorubicin and Etoposide resistant MCF-7 cell lines, APOBEC3B expression was approximately five-fold increased (23% and 24% respectively) with higher signal intensity (1.92 and 1.44 signal/cell, respectively) compared to drug sensitive MCF-7 cell line (5%, 1.00 signal/cell) with statistical significance. The increase of APOBEC3B expression in Docataxel resitant and Paclitaxel resistant MCF-7 cell lines was not very high. In conclusion, APOBEC3B expression was increased in some population of tumor cells of drug resistant cell lines. At least for some drugs, APOBEC3B expression may be related to drug resistance, subjecting to some tumor cells to frequent mutation. (C) 2016 Elsevier Masson SAS. All rights reserved.