In order to simplify the comparison of data from different laboratories, a uniform numbering system for the'hammerhead'ribozyme is proposed. The three RNA helices (numbered I, II and II) are specially arranged in Figure 1 as originally proposed (1), but alternateformats (2) are alsouseful. In Figure 1 we have arbitrarily chosen a hammerhead with a loop 1, a loop 3 and an open helix II ending with dangling nucleotides. Starting with the nucleotide 3'to the cleavage site, the seventeen nucleotides in the central core are numbered in a clockwise fashion. Six nucleotides in the core (I. 1, 2.1, 10.1, 11.1, 15.1 and 16.1) also receive a decimal to indicate that they are the first nucleotide in the helix. Subsequent residues in the three helices are numbered with sequential decimals extending outward from the core. Unless bulges interrupt the helix, this will result in nucleotides with the same decimal pairing with one another (1.3 pairs with 2.3, 10.4 with 11.4, etc.). If the helices form a hairpin, the single stranded loop is named according to the helix number and the residues are given decimals starting from the 5'side of the loop. This system allows a simple definition of the basic features of any hammerhead. The numbering of two specific hammerheads (ASBV and the complex of a designed ribozyme with its target RNA) are given in Figure 2A and 2B. Following tRNA nomenclature (3), the phosphate 5'to a nucleotide receives the number of the nucleotide. Thus, cleavage occurs betweennucleotides 17 and 1.1 and results in phosphate 1.1 attached to ribose 17 as a 2', 3'cyclic phosphodiester. Mutations are named by listing the original nucleotide, the position number and the mutation. For example, a change of the G residue at position 5 to an A residue would be denoted G5A. A similar method will be usedto describe backbone modifications so that a rl7d hammerhead containsa 2'deoxy-nucleotide at the cleavage site (and thus will not cleave). The authors agree to use this nomenclature in future publications and urge the community to do so as well.