motifs that may represent structural units of the titin PEVK spring. The number of PEVK motifs in the titin gene appears to have increased during evolution, apparently modifying the genomic region responsible for titin's spring properties. Isoforms A number of titin isoforms are produced in different striated muscle tissues as a result of alternative splicing. All but one of these isoforms are in the range of ~27,000 to ~36,000 amino acid residues in length. The exception is the small cardiac novex-3 isoform, which is only 5,604 amino acid residues in length. The following table lists the known titin isoforms: Structure Titin is the largest known protein; its human variant consists of 34,350 amino acids, with the molecular weight of the mature "canonical" isoform of the protein being approximately 3,816,030.05 Da. Its mouse homologue is even larger, comprising 35,213 amino acids with a molecular weight of 3,906,487.6 Da. It has a theoretical isoelectric point of 6.02. The protein's empirical chemical formula is C169,719H270,466N45,688O52,238S911. It has a theoretical instability index (II) of 42.38, classifying the protein as unstable. The protein's in vivo half-life, the time it takes for half of the amount of protein in a cell to break down after its synthesis in the cell, is predicted to be approximately 30 hours (in mammalian reticulocytes). The Titin protein is located between the myosin thick filament and the Z disk. Titin consists primarily of a linear array of two types of modules, also referred to as protein domains (244 copies in total): type I fibronectin type III domain (132 copies) and type II immunoglobulin domain (112 copies). However, the exact number of these domains is different in different species. This linear array is further organized into two regions: N-terminal I-band: acts as the elastic part of the molecule and is composed mainly of type II modules. More specifically the I-band contains two regions of tandem type II immunoglobulin domains on either side of a PEVK region that is rich in proline (P), glutamate (E), valine (V) and lysine (K). C-terminal A-band: is thought to act as a protein-ruler and is composed of alternating type I (Fn3) and II (Ig) modules with super-repeat segments. These have been shown to align to the 43 nm axial repeats of myosin thick filaments with immunoglobulin domains correlating to myosin crowns. The C-terminal region also contains a serine kinase domain that is primarily known for adapting the