Likewise, knock-in mice where the MOR is replaced by a mutant receptor which cannot be phosphorylated by GRKs (MOR 11S/T-A) are also reported to show improved analgesia and reduced analgesic tolerance but no difference in respiratory depression in response to morphine (Kliewer et al., 2019). The gene discussed is OPRM1; the disease is Respiratory insufficiency due to muscle weakness.