The present invention encompasses methods for producing a modified polynucleotide sequence that comprises a (e.g., one or more) phosphorothiolate linkage, methods for determining a polynucleotide sequence comprising a (e.g., one or more) phosphorothiolate linkage, and methods for separating forward and reverse extension products that comprise a (e.g., one or more) phosphorothiolate linkage. The invention also encompasses kits for producing and/or determining the sequence of a modified polynucleotide that comprises a (e.g., one or more) phosphorothiolate linkage.