AbstractOverexpression of dominant oncogenes and the loss of tumor suppressor genes are basic genetic events in the acquisition of the malignant phenotype. The erb‐b2 receptor tyrosine kinase 2 (ERBB‐2) proto‐oncogene is overexpressed in 20‐30% of human breast cancers. The StAR related lipid transfer domain containing 13 gene (STARD13), also known as Deleted in Liver Cancer‐2 (DLC‐2), maps to chromosome band 13q12.3 and is frequently downregulated in human cancers, including 72% of breast cancers. It encodes a RhoGAP protein with sterile α motif (SAM) and StAR‐related lipid transfer (START) domains. The objective of this study was to determine if loss of Stard13 plays a role in mammary tumor progression using transgenic mice expressing the activated ErbB‐2 (Neu) oncogene and Cre recombinase (NIC) in mammary epithelium under transcriptional control of the murine mammary tumor virus (MMTV) promoter (MMTV‐NIC). These mice were crossed with a conditional Stard13 knockout mouse (floxed exon 3), resulting in simultaneous Neu expression and Stard13 deletion, specifically in the mammary epithelium. We found that loss of Stard13 did not alter tumor growth nor significantly modify overall survival and tumor free survival. However, there was an increase in the total number of lung metastases in the Stard13 heterozygous or homozygous mice compared with the parental MMTV‐NIC strain. Altogether our results indicate that Stard13 acts as a metastasis suppressor rather than a tumor suppressor gene, in Neu oncogene induced mammary tumorigenesis.