diff --git a/inst/doc/MLplay.bib b/inst/doc/MLplay.bib
index 42dafa1..f91695c 100644
--- a/inst/doc/MLplay.bib
+++ b/inst/doc/MLplay.bib
@@ -37,6 +37,14 @@ @article{Noble:2006br
 pages = {1565--1567}
 }
 
+@article{Sonnenburg:2007wu,
+author = {Sonnenburg, S. and Rätsch, G and Rieck, K.},
+journal = {Large Scale Kernel Machines},
+title = {{Large scale learning with string kernels}},
+month = {},
+year = {2007}
+}
+
 @article{Leslie:2004kt,
 author = {Leslie, Christina S and Eskin, Eleazar and Cohen, Adiel and Weston, Jason and Noble, William Stafford},
 journal = {Bioinformatics},