I am applying for PhD positions in the US, and need to write statement of purpose. I am currently in a Masters program, and I believe that I have a lot of interesting and major research experiences (which even lead to publications) during my Masters program which I want to talk about in my statement. However, I was wondering how much I should talk about my undergraduate experiences. I do have some interesting research experiences, but they did not lead to any publications. Given the fact that I only have a page or two for the essay, any suggestions on how to fit my experiences from Masters and undergraduate program?