The double tudor domain is part of the human Lysine-specific demethylase 4A and recognizes the methylation state of histones. Histones are proteins found in the nucleus of cells where they organize the DNA by wrapping it around them selves. They are intimately involved in the regulation of gene expression through their methylation state. This proteins allows the cells to "sense" the methylation and is part of Lysine-specific demethylase 4A which is able to remove the methylation from histones allowing them to bind to DNA again. This would switch of any genes in the affected stretch of DNA.