Transcription of the AUX/IAA family of genes is rapidly induced by the plant hormone auxin, but evidence that AUX/IAA genes mediate further responses to auxin has been elusive. Changes in diverse auxin responses result from mutations in the Arabidopsis AXR3 gene. AXR3 was shown to be a member of the AUX/IAA family, providing direct evidence that AUX/IAA genes are central in auxin signaling, Molecular characterization of axr3 gain-of-function and loss-of-function mutations established the functional importance of domains conserved among AUX/IAA proteins.