ZIP9 membrane androgen receptor was first discovered in Atlantic croaker (Micropogonias undulatus) brain, ovary and testicular tissues and named "AR2" in 1999, together with another androgen receptor which was found only in brain tissue, and it was named "AR1" in that time. AR1 and AR2 were first thought to be nuclear androgen receptors (nAR), however, further studies on their biochemical and functional features in 2003 illustrated that they were involved in non-genomic mechanisms in the plasma membrane of the cells and were membrane androgen receptors. In 2005, the similarities between the nucleotide and amino acid sequences of AR2 and ZIP family proteins were discovered in other vertebrates, suggesting that AR2 is from this family of proteins. A study in 2014 utilised the latest research technologies to clone and express a particular cDNA of the female Atlantic croaker ovaries, which encoded a protein showing the characteristics of the canonical isoform of ZIP9, as a novel membrane androgen receptor(mAR).
