An invariant residue, represented as pink circle, in the middle of each of the S1-S6 and P1-SF-P2 segments, is selected as the benchmark in the generic numbering system and numbered 30.
The benchmark is selected as the most conserved residue in the middle of a structural segment and assigned the residue number 30 to ensure that all residues would have a positive number. The term "identifier" refers to the structural position relative to the benchmark residue, depicted as the following: the one-letter code for an amino acid is followed with, in subscript, the segment number and repeat number, a comma, and the residue number relative to the benchmark residue.
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