We demonstrate that REF6/JMJ12 (RELATIVE OF EARLY FLOWERING 6/Jumonji domain-containing protein 12) is an H3K27me3 and H3K27me2 demethylase. Plants overexpressing REF6/JMJ12 resemble mutants defective in H3K27me3-mediated gene silencing. Genetic interaction tests indicate that REF6/JMJ12 acts downstream of H3K27me3 methyltransferases. Moreover, loss of REF6/JMJ12 leads to ectopic and increased H3K27me3 and decreased mRNA expression of a large spectrum of genes involved in development and hormone responses to stimuli. Expression array to identify differentially expressed genes in ref6-1. For genome-wide expression analysis of ref6/jmj12, two replicates of Col and ref6/jmj12 samples (RNA from 10-day-old whole seedlings on MS plates in long day conditions at 23 degrees) were analyzed on an Affymetrix ATH1 chip by an Affymetrix service facility (CapitalBio Corporation) according to the manufacturer's protocols. Genes showing a 20.6-fold change with a q-value < 0.05 were considered as differentially expressed.