To identify sequence context features affecting somatic mutation accumulation in GC, we considered 1-mer, 3-mer, and 5-mer nucleotide motifs centered at the mutated site, as well as 1-bp and 2-bp left/right flank motifs of the site. All nucleotide context features were grouped into reverse compliment pairs. As indels tend to occur in poly-monomer sequences, especially poly-A and poly-T sequences, we used the presence of poly-A, poly-T, poly-G, and poly-C sequences at the indel sites as features in the indel background mutation model.