A package to compress DNA sequence, using a reference genome. DNAzip uses a series of compression techniques when, taken together, reduces the size of a single genome by orders of magnitude. It permits sending genome by mail, for example. This technique does require a reference human genome (∼3 GB) and a reference SNP map (∼1.2 GB), but this cost is amortized over the total number of genomes. DNAzip runs in time that scales only linearly to the number of variations.