This disclosure includes compositions comprising different synthetic polynucleotides that can be used in determining whether a given gene mutation (e.g., a cancer-associated mutation) is present in a target polynucleotide and/or determining an abundance of a target polynucleotide (e.g., by amplifying and sequencing the different synthetic polynucleotides and the target polynucleotide). In some aspects, the synthetic polynucleotides each comprise a barcode, which can be used to differentiate identical synthetic polynucleotides and differentiate the synthetic polynucleotides and the target polynucleotide.