I am new to the mammalian expression system. Recently, I was working to make specific point mutation on the mouse genome with CRISPR. I have made a HDR donor construct "gene 5' Arm----PGK---Puro----3' Arm" but without Poly A after Puro. My question is whether I can use puromycin to screen my ESCs. I am worried about Puro does't work without PolyA. Thank you so much in advance.