eer4 was isolated as an Arabidopsis mutant with an extreme response to ethylene in dark-grown seedlings that was also found to have partial ethylene insensitivity at the level of ethylene-dependent gene expression, including ERF1. Subsequent cloning of eer4 revealed an inappropriate stop codon in a previously uncharacterized TFIID-interacting transcription factor homologous to human TAF12 and yeast TAF61. Genetic and pharmacological analysis demonstrated that the eer4 phenotype is strictly ethylene dependent in seedlings, yet a double mutant with the partially ethylene-insensitive Arabidopsis mutant, ein3-1, had restored ethylene responsiveness, indicating that eer4 also regulates a previously unknown resetting or dampening mechanism for the ethylene signalling pathway. Consistent with the absolute requirement of EER4 for ERF1 expression, biochemical analysis showed that EER4 is localized to the nucleus where it probably recruits EIN3 and probably other transcription factors along with components of the TFIID complex for expression of a subset of genes required for either manifestation or subsequent dampening of the response to ethylene.