Like parvalbumins, SCPs seem to be more abundant in fast contracting muscles, but no functional relationship can be established from this distribution. The sarcoplasmic calcium-binding proteins are abundant in the muscle of arthropods, mollusks, annelids, and protochordates. This protein has three functional calcium-binding sites; potential site 4 has lost affinity for calcium. There are 7 different SCP's in amphioxus. The sequence shown here is that of SCP I.