An extra rib at the cervicothoracic border with the distal extremity rounded, length less than one third of the length of the ossified portion of the first thoracic rib and no costal cartilage distal. (Makris S, Solomon HM, Clark R, Shiota K, Barbellion S, Buschmann J, Ema M, Fujiwara M, Grote K, Hazelden KP, Hew KW, Horimoto M, Ooshima Y, Parkinson M, Wise LD. Terminology of developmental abnormalities in common laboratory mammals (Version 2). Part B. Birth Defects Res B Dev Reprod Toxicol. 2009 Aug;86(4):227-327.) [from
NCI]