**2.4 Thyrotropin beta subunit gene (***TSHB***)**

The porcine *TSHB* gene has at least two exons and one intron, and is 939 bp in length. Its product consists of a signal peptide of 24 amino acid residues, a mature beta subunit protein of 112 residues and an additional extension of six amino acid residues at the carboxyl terminus (Hirai et al., 1989). The TSH beta subunit noncovalently links to the alpha subunit by wrapping with its "seat belt" structure around the alpha subunit's long loop αL2.

TSH β subunit is responsible for the hormone specificity, and its translation rate directly determines the secretion and synthesis rates of the hormone TSH. Additionally, it has been reported that mature Chinese meishan pigs had 3 fold greater expressions of *TSHB* gene and greater plasma TSH concentrations than mature Western white composite pigs (Li et al., 1996).

### **2.5 Thyrotropin receptor gene (***TSHR***)**

The receptor of the thyrotorpin,TSHR, belongs to the glycoprotein hormone receptor (GPHR) family which is a subset of the G-protein coupled receptor (GPCR) superfamily. It contains a seven-transmembrane domain and a large ectodomain composed of many leucine-rich repeats forming TSH-binding surface (Farid & Szkudlinski, 2004).

The mRNA sequence of the porcine *TSHR* gene has been cloned in 2003 (Igarashi & Nagata, 2003) and characterized to containing an open reading frame coding 764 amino acids. However, as the genomic sequence of porcine *TSHR* gene is still unavailable so far, the genomic structure of the gene is not clear. Of human, the *TSHR* gene was found to be 190 kb in length, and contains ten exons and nine introns, of which all the first nine exons encodes the large extodomain while the last exon is in charge of the seven transmembrane domains and the intracellular tail.
