In order to determine the localization and abundance of MaTAR25, we performed single-molecule RNA fluorescence in situ hybridization (smRNA-FISH) to detect MaTAR25 RNA transcripts within MMTV-PyMT (luminal B) and MMTV-Neu-NDL (HER2/neu+) primary mammary tumor cells. This evidence concerns the gene ERBB2 and breast cancer.