Genetic approaches to identify molecular components of transcriptional gene silencing (TGS) in plants have yielded several Arabidopsis thaliana mutants and identified the first genes involved. All mutations found affect the maintenance of silencing and reactivate silent genes in trans. The mutations fall into two categories: ddm1 and hog release silencing in association with decreased levels of DNA methylation, while sil and mom reactivate genes without changing the methylation state. While plants homozygous over several generations for hog, sil or mom exhibit no morphological changes, ddm1-type mutants accumulate developmental abnormalities. The mutants indicate that TGS in plants is controlled by several genetic components and possibly by multiple independent pathways. The DDM1 gene was assigned to the SWI2/SNF2 gene family of chromatin-remodelling proteins, the MOM gene is a novel protein and the other loci have not yet been characterized.