Model of spectrin products resulting from the α-spectrin St Claude allele. Each block represents a 106 residue segment, except block 10, which represents the SH3 domain. The nucleation site involves repeats 19-22 of α chain and repeats 1-4 of β chain. The 277-kD mutant, product of the mRNA species with the skipping of exon 20, is deleted of helix B₉ from residue G₉₂₆ to residue Q₉₅₈ (cross-hatched segment). The 108-kD α chain, hypothetical product of the mRNA species with the 12 nt insert, ends at residue G₉₂₆ . This truncated α chain does not contain the nucleation site allowing its association with β chain and its recruitment to the membrane. The upper part of the diagram represents a magnification of the triple helix structure with helices C₉ , A₉ , and B₉ of repeat 9 and helices C₁₁ and A₁₁ of repeat 11, around the SH3 domain (segment 10). (Adapted and reprinted with permission.22 )