Ethylene receptor expression in the single loss-of-function ethylene receptor mutants. Northern analysis was performed to determine the expression level of the ethylene receptors in receptor loss-of-function mutants. Total RNA from untreated leaves from 4-week-old plants was collected and 10 μg from each sample was electrophoretically separated and analyzed for expression of ETR1, ERS1, ETR2, and EIN4, along with Arabidopsis tubulin TUA3, which was used as a loading control.