Interacts with RAP2A. Isoform 2 is expressed at high levels in brain, thymus, ovary, testis, leukocyte, liver, small intestine and prostate. Isoform 1 is expressed in the brain, testis and adrenal gland. It is activated in tumorigenic breast cancer cell lines and in the primary tumor of breast cancer patients. Activation also correlates with metastatic lymph node invasion and can be detected in metastatic epithelial cells from the lymph nodes and in the bone marrow of patients. Belongs to the RUNDC3 family.