Abstract: Provided are novel selectable markers and uses thereof. Specifically, provided are uses of nucleotide sequences encoding a glutamine synthetase (GS) derived from Alligator, green anole, or spotted gar as selectable markers for identifying genomic loci with high transcriptional activity or host cells having high productivity of a protein of interest, and/or for accelerating the identification process. Related methods of screening, methods of production, and expression systems are also included.