**3.1 The amino acid sequences of human PRL prohormone and mature PRL**

In human pituitary, the PRL prohormone is synthesized in the ribosome, with 227 amino acids (position 1–227; 25.9 kDa), containing a signal peptide (position 1–28) (**Table 1**), which was assigned with Swiss-Prot accession No. P01236. However, the mature human PRL only contains 199 amino acids (position 29–227; 22.9 kDa), which removed the signal peptide (position 1–28), and secreted into the circulation system to bind to its target organ for exerting PRL function.
