Tudor domains are found in many eukaryotic organisms and have been implicated in protein-protein interactions in which methylated protein substrates bind to these domains. For example, the Tudor domain of Survival of Motor Neuron (SMN) binds to symmetrically dimethylated arginines of arginine-glycine (RG) rich sequences found in the C-terminal tails of Sm proteins. The SMN protein is linked to spinal muscular atrophy. Another example is the tandem tudor domains of 53BP1, which bind to histone H4 specifically dimethylated at Lys20 (H4-K20me2). 53BP1 is a key transducer of the DNA damage checkpoint signal.