Jeongbin Park¹, Liam Childs¹, Daesik Kim², Gue-Ho Hwang³, Sunghyun Kim², Sang-Tae Kim², Jin-Soo Kim^2,4,* & Sangsu Bae<sup>3,5,*

1</sup>Division of Theoretical Bioinformatics, German Cancer Research Center (DKFZ), Heidelberg, Germany. ²Center for Genome Engineering, Institute for Basic Science, Seoul, South Korea. ³Department of Chemistry, Hanyang University, Seoul, South Korea. ⁴Department of Chemistry, Seoul National University, Seoul, South Korea. ⁵Institute of Nano Science and Technology, Hanyang University, Seoul, South Korea.

^*Correspondence should be addressed to S.B. or J.-S.K..

To the Editor:
We recently reported Digenome-seq (digested genome sequencing), a method for in vitro identification of potential off-target sites, and we evaluated the specificity of CRISPR–Cas9 (refs. 1,2) and CRISPR–Cpf1 (ref. 3) endonuclease by whole-genome sequencing. Digenome-seq pinpoints the exact location of double-strand break (DSB) sites by recognizing specific patterns of aligned reads. However, the analysis pipeline presented in our previous report required extensive manual interaction and produced several large intermediate files, resulting in a long running time. Here, we present a redesigned analysis tool for Digenome-seq data that runs on web browsers. The core algorithm of the tool is written in C++ and compiled to asm.js (http://asmjs.org/), a preoptimized subset of JavaScript. Users can instantly perform the complete analysis in an ordinary web browser (Supplementary Note 1) with fast execution speed without uploading any data to a server and without local tool installation. In our benchmark, the full analysis for 100 GB of BAM file took 3 h for whole analysis on Intel i5 3570k central processing unit in a single thread.