Breast cancer (BC) is the second most common tumor worldwide, and is the first in incidence in women. Nowadays, there are only a few pre-clinical syngeneic murine models for BC study. M46 tumor spontaneously appeared in a BALB/c female of the Instituto Roffo Animal´s Facility. Tumor was maintained through subcutaneous (sc) transplantation and cryopreservation. The objective of this study was to characterize the novel M46 breast tumor to establish a new pre-clinical murine model. M46 tumor was transplanted with a trocar needle into the sc of females and males BALB/c mice. Tumor grew in all animals, being euthanized after 20 days of inoculation. Tumor volume reached 1029 ± 367.3mm3 in female and 1040 ± 367.4mm3 in male (mean ± SD). Tumor, lungs and liver were fixed to histological analysis and metastasis quantification. The histopathological diagnostic determined that M46 is a High Grade (GIII) invasive carcinoma, with a Nottingham Score of 8 (GH3, GN3, GM2). It has sections with atypical neoplastic proliferation consisting of sheets of epithelial cells with anisodiskaryosis and numerous atypical mitotic figures. M46 presents areas of necrosis and hemorrhage, with a few areas of associated stroma accompanied by mild inflammatory infiltrate. These pathological features are related with an aggressive tumor associated with the general disease that we observed in our mice. M46 tumor developed spontaneous lung metastasis in 100% of mice, with 11 (2-100) and 9 (2-18) metastasis per lung (median and range) in female and male respectively. The incidence of liver metastasis was 55% in female and 70% in male with a median of 1 per liver. There are no significant differences between females and males in the tumor size, tumor growth kinetics, lung metastasis and liver metastasis develop. In conclusion, our findings establish M46 as a novel pre-clinical murine model of invasive and metastatic BC. This model holds promise as a valuable tool for further research into this disease