Mutations in the SUPERMAN gene affect flower development in Arabidopsis. Seven heritable but unstable sup epi-alleles (the clark kent alleles) are associated with nearly identical patterns of excess cytosine methylation within the SUP gene and a decreased level of SUP RNA. Revertants of these alleles are largely demethylated at the SUP locus and have restored levels of SUP RNA. A transgenic Arabidopsis line carrying an antisense methyltransferase gene, which shows an overall decrease in genomic cytosine methylation, also contains a hypermethylated sup allele. Thus, disruption of methylation systems may yield more complex outcomes than expected and can result in methylation defects at known genes. The clark kent alleles differ from the antisense line because they do not show a general decrease in genomic methylation.