Each sequence is then compared to a reference sequence through read alignment. Reference sequences are considered “normal” in that there are no known significant variants; however, there is no real “normal” sequence, especially for human DNA. Variations from the reference may be the majority allele in the popu-lation, with the reference sequence carrying the minor allele. For human genome sequencing, reference genome hg19 was frequently used and reference genomes are updated periodically. Reference sequences are free of known disease-related alleles, at least those found in the targeted panels.