Described is a DNA molecule comprising an open reading frame sequence encoding a selectable marker polypeptide, wherein the DNA molecule in the coding strand comprises a translation start sequence for the selectable marker polypeptide having a GTG or TTG start codon, and wherein the ORF sequence that encodes the selectable marker protein has been mutated to replace at least half of its CpG dinucleotides as compared to the native ORF sequence that encodes the selectable marker protein. Further provided are such DNA molecules wherein the ORF sequence that encodes a selectable marker polypeptide is part of a multicistronic transcription unit that further comprises an open reading frame sequence encoding a polypeptide of interest. Also described are methods for obtaining host cells expressing a polypeptide of interest, wherein the host cells comprise the DNA molecules described herein. Further provided is the production of polypeptides of interest, comprising culturing host cells comprising the DNA molecules described herein.