The kinase activity of CTR1 becomes activated and phosphorylates the neighbouring EIN2.[1] As long as EIN2 remains highly phosphorylated, it remains inactive and there never is an ethylene signal relay. In ETR1, the receptor histidine kinase is required for binding with EIN2.[29] RTE1 can bind to and activate ETR1 independent of CTR1.[30] There is evidence that cytochrome b5 aids or acts similar to RTE1.[31]