K.J. Hertel, A. Pardi, O.C. Uhlenbeck, M. Koizumi, E. Ohtsuka, S. Uesugi, R. Cedergren, F. Eckstein, W.L. Gerlach, R. Hodgson, R.H. Symons, Numbering system for the hammerheadNucleic Acids Research. ,vol. 20, pp. 3252- 3252 ,(1992) , 10.1093/NAR/20.12.3252