**2. Kisspeptin**

The Kiss1 gene encodes the neuropeptide kisspeptins. They, originally identified as metastasis suppressors, were later found to play a central regulatory role in reproduction [18]. The gene is located on human chromosome 1q32. This gene contains 2 non-expressed and 2 partially expressed regions and four exons, assembling the leader peptide consisting of 145 amino acids [19]. This precursor protein transforms into various active forms of kisspeptin with lengths of 54, 14, 13, and 10 amino acids through various post-translational modifications (**Figure 1**) [20]. These forms belong to the RF-amide peptide hormone family, which is closely associated with energy metabolism and reproduction. The members of the RF-amide peptide hormone family contain the common Arg-Phe-NH2 moiety at the C-terminal [21].

Kisspeptin receptors were first discovered in 2001 in studies about cancer, and they were named as the GPR54. Today, they are identified as the Kiss1r [9]. Kiss1r is a 396-amino acid receptor and a member of G protein-coupled receptors. The minimum length required to activate GPR54 is a 10-amino acid carboxyl terminal sequence (Kisspeptin-10) [22].

#### **Figure 1.**

*Amino acids sequence of human kisspeptin isoforms [23, 24].*
