Like parvalbumins, SCPs seem to be more abundant in fast contracting muscles, but no functional relationship can be established from this distribution. SCPs from crayfish, lobster, and shrimp are polymorphic dimers; three isotypes (alpha-alpha, alpha-beta, and beta-beta) have been identified. Expressed in tail muscle (at protein level). Causes an allergic reaction in human. Binds to IgE of patients allergic to shrimp (PubMed:19523674, PubMed:20471069, PubMed:22192087). Natural boiled protein binds to IgE in 59.6% of the 52 shrimp-allergic patients tested. Recombinant protein binds to IgE in 64.5% of the 31 patients tested allergic to the natural protein. 85% of the patients binding the recombinant protein are children (PubMed:19523674). Epitope diversity and the frequency and intensity of IgE-binding is significantly more pronounced in children than in adults (PubMed:20471069). Patients (children or adults) with positive double-blind placebo-controlled food challenge (DBPCFC) to shrimp have a more frequent, intense and diverse recognition of the epitopes of this protein than those with negative challenge (PubMed:22192087). Recombinant protein activates rat basophilic leukemia (RBL) cells expressing human immunoglobulin epsilon Fc receptor type 1 and releases beta-hexosaminidase from the cells (PubMed:19523674). The sarcoplasmic calcium-binding proteins are abundant in the muscle of arthropods, mollusks, annelids, and protochordates.