Histone methyltransferases are enzymes that add methyl groups to specific amino acids on histone proteins, which are key components of chromatin. This process is crucial for regulating gene expression, as the addition of methyl groups can either activate or repress the transcription of genes, depending on the specific histone and the location of the methylation. These enzymes play a significant role in various biological processes, including cell differentiation, development, and DNA repair. Dysregulation of histone methyltransferases has been linked to several diseases, including cancer, highlighting their importance in maintaining proper cellular function and gene regulation.