However, duplication events in teleosts are common, and some teleostean genes still exist in duplicate today, as in the class-I a-helical cytokine family (Huising et al., 2006b,c; 2005). Moreover, more recent genome duplications, or tetraploidisation events in common carp and salmonids result in up to four leptin paralogues in these species (Angotzi et al., 2013; Huising et al., 2006a; Rønnestad et al., 2010). One can argue, that these gene pairs must have differentiated in functionality or in temporal (ontogeny) and spatial (morphology) expression patterns, otherwise one of the two would have disappeared over time