RNA interference (RNAi) is a posttranscriptional gene silencing mechanism used to study gene functions, knock down viral genes, and treat diseases therapeutically. However, an 'off-target effect' deteriorates its specificity and applicability. Complete off-target effects can only be characterised by examining each gene in a genome, which is too expensive to conduct experimentally and motivates a computational study. To simulate the sequence matching between an siRNA and its target mRNA allowing for mismatches, G-U wobbles and bulges, we propose string kernels and develop their efficient implementations for off-target detection. We evaluate RNAi specificities in Schizosaccharomyces pombe, Caenorhabdithis elegans, and human genomes.