According to the family tree edited by Yoshikazu (義和) SAKUMA in Sendai Domain, Tanenaga KOKUBUN was the first son of Tanemichi KOKUBUN, and his mother was a daughter of Tsunenobua USUI (臼井常信).
江戸時代に仙台藩の佐久間義和が編纂した系図では、国分胤長は国分胤通の長子、母は臼井常信の女。