Schematic representations of the gene and primary protein structures of two pairs of related SET-domain histone methyltransferases in the SUV39 family. (a) Human SUV39H1 (gene, mRNA and protein); (b) human SUV39H2 (gene and mRNA for comparison with SUV39H1); (c) human G9a (gene, mRNA and protein); (d) human GLP1 (EuHMT1; mRNA for comparison with G9a; the gene structure is not shown because of the large size of the intron). Black boxes in the genes and white boxes in the mRNAs denote exons; numbers above each gene are exon numbers; numbers within exons indicates their size in nucleotides; thin lines in the genes indicate the introns and untranslated regions of the first and the last exons (these are shown to scale with the length of the exons except where lines are broken). (a,c) Protein structures are shown on the same scale as the coding region of the corresponding mRNA, so that the corresponding exons for each protein domain can be directly aligned. Domains are indicated above protein structures, and the number of conserved cysteines (Cys) in each domain is also shown. Abbreviations: Chromo, chromodomain; E-rich, glutamine-rich domain; KR-rich, domain rich in lysine and arginine; NRSF-binding, a domain involved in binding neuron-restrictive silencing factor/repressor element 1 silencing transcription factor. (c,d) The intron-exon junctions indicated with asterisks are those that differ between G9a and GLP1.