Based on this principle, reversibly binding photochromic ligands have been developed for various GPCRs, including CXCR3,[12] μOR,[13] mGlu5,[14] CB1,[15] H3R,[16] α2AAR,[17] and Y4R.[18] A photoswitchable antagonist has also been reported for the β2AR, an important therapeutic drug target for the treatment of airway diseases such as asthma or chronic obstructive pulmonary disease (COPD).[19]. This evidence concerns the gene HRH3 and chronic obstructive pulmonary disease.