Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.
Q: Context: In a phase I study, Gilead Sciences' PI3K- inhibitor idelalisib produced rapid tumor shrinkage-a response that lasted for 17 months on average-in about half of the 54 patients with chronic lymphocytic leukemia who participated.
Question: What is the target protein of the drug Idelalisib?
A:
phosphoinositol-3 kinase delta isoform