Purified BSA (NEB# B9001) is not designed to be used as a molecular weight marker. Our Protein Marker , Broad range (NEB# P7702) uses BSA as one of its marker bands. This BSA used in the protein marker is selected for size and the absence of breakdown products. The molecular weight of BSA is 66,409 daltons. Purified BSA (NEB# B9001) run on SDS PAGE will give a major band at this size. We do not guarantee that it will give a single sharp band when used in this manner.