You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Israel

Context:
Israeli universities are among 100 top world universities in mathematics (Hebrew University, TAU and Technion), physics (TAU, Hebrew University and Weizmann Institute of Science), chemistry (Technion and Weizmann Institute of Science), computer science (Weizmann Institute of Science, Technion, Hebrew University, TAU and BIU) and economics (Hebrew University and TAU). Israel has produced six Nobel Prize-winning scientists since 2002 and has been frequently ranked as one of the countries with the highest ratios of scientific papers per capita in the world. Israel has led the world in stem-cell research papers per capita since 2000.

Question:
Israel leads the world in what research papers per capita?

stem-cell research