The extracellular loop 2 (ECL2) of the human histamine H4 receptor substantially contributes to ligand binding and constitutive activity.

The mutation of F169 alone and, even more pronounced, the mutation of both F169 (ECL2) and S179 5.43 (numbering according to the Ballesteros nomenclature [ 41 ]) into the corresponding amino acids of the mouse and rat H 4 R orthologs (F169V, S179M, S179A) resulted in a highly significant reduction of the constitutive activity [ 9 ].

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