https://academic.oup.com/bioinformatics/article-pdf/20/11/1682/850924/bth141.pdf (Publisher version)
Saigo, H., Vert, J.-P., Ueda, N., & Akutsu, T. (2004). Protein homology detection using string alignment kernels. Bioinformatics, 20(11), 1682-1689. doi:10.1093/bioinformatics/bth141.