This family exemplifies a case of novel gene evolution. The case in point is the arms-race between plants and their infective insective herbivores in the area of the glucosinolate-myrosinase system. Brassicas have developed the glucosinolate-myrosinase system as chemical defence mechanism against the insects, and consequently the insects have adapted to produce a detoxifying molecule, nitrile-specifier protein (NSP). NSP is present in the small white butterfly Pieris rapae. NSP is structurally different from and has no amino acid homology to any known detoxifying enzymes, and it appears to have arisen by a process of domain and gene duplication of a sequence of unknown function that is widespread in insect species and referred to as insect-allergen-repeat protein. Thus this family is found either as a single domain or as a multiple repeat-domain [3].

1: Immunologic characterization of a recombinant American cockroach (Periplaneta americana) Per a 1 (Cr-PII) allergen. Wang NM, Lee MF, Wu CH; Allergy 1999;54:119-127. PMID:10221434

2: Cloning of the American cockroach Cr-PII allergens: evidence for the existence of cross-reactive allergens between species. Wu CH, Wang NM, Lee MF, Kao CY, Luo SF; J Allergy Clin Immunol 1998;101:832-840. PMID:9648712

3: Evolutionary origins of a novel host plant detoxification gene in butterflies. Fischer HM, Wheat CW, Heckel DG, Vogel H; Mol Biol Evol. 2008;25:809-820. PMID:18296701