ARRB2 and respiratory depression: Beyond this, several important future studies to exhaust other methodological limitations include the measurement of respiratory depression in Arrb2-/- and littermates with other abused opioids like oxycodone and heroin, to temporally or spatially delete Arrb2 using Arrb2flox/flox alleles to overcome possible compensatory mechanisms, and the study of biased agonists in models where potent opioids like fentanyl are lethal.