Three distinct thymopoietins have been characterized that are splice variants involving similar N-termini, but different C-termini with molecular weights of 75,51, and 39 kDa. mRNAs are expressed in various tissues but are most abundant in adult thymus and fetal liver. These proteins may have unique functions and have subcellular locations in different compartments. It is of interest that expression of human myotonin protein kinase cDNA in COS cells encodes a protein with a thymopoietin-like domain. A thymopoietin-related peptide has been cloned from a thymus cDNA library and contains the active site of thymopoietin (amino acids 32-36). This protein and thymopoietin differ in only one amino acid residue from bovine thymopoietin and in five residues from bovine thymopoietin I. Thymopoietin related peptide is a novel peptide counterpart of bovine thymopoietin.

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