The reason of this delay lies in a very low amino acid sequence conservation; depending on the
fish species between 13–25% homology compared with human leptin. Despite a dramatic difference in primary sequence, true orthology between fish and other vertebrate leptins was demonstrated based on conserved gene structure, stable clustering with other vertebrate leptin genes in phylogenetic analyses and conserved tertiary structure when modelled with the human leptin structure [PDB entry 1AX8 (Zhang et al., 1997)] as template