From what I read in the past two days in this thread, I think Dr. Menon is putting more faith in the p21 as a biomarker (for Kevetrin's activity at least) than some of his professional colleagues now are in general.
btw, I wonder how hard it would be to test for PUMA. Now that there's research showing the PUMA/p21 ratio could be relevant, I'd love it if they tested for PUMA too.