The present invention provides methods, compositions, kits, systems and apparatus that are useful for isolating nucleic acid molecules from a sample. In particular, the methods generally relate to normalizing the concentration of target nucleic acid molecules from a sample. In one aspect, the invention relates to purifying a primer extension product from a primer extension reaction mixture. In some aspects, nucleic acid molecules obtained using the disclosed methods, kits, systems and apparatuses can be used in various downstream processes including nucleic acid sequencing.