A computer-implemented method for classifying alignments of paired nucleic acid sequence reads is disclosed. A plurality of paired nucleic acid sequence reads is received, wherein each read is comprised of a first tag and a second tag separated by an insert region. Potential alignments for the first and second tags of each read to a reference sequence is determined, wherein the potential alignments satisfies a minimum threshold mismatch constraint. Potential paired alignments of the first and second tags of each read are identified, wherein a distance between the first and second tags of each potential paired alignment is within an estimated insert size range. An alignment score is calculated for each potential paired alignment based on a distance between the first and second tags and a total number of mismatches for each tag.