Wind over mountains mostly blows downhill because that is the path of least resistance. But in some valleys, the air curls into small eddies. Mutations in human DNA behave in much the same way.
Such cycles break the equilibrium in which every replacement is balanced by its reverse. They are weak, but they show up across different human populations. The strongest eddies occur at CpG sites, where cytosine precedes guanine. This is because methylated cytosine can lose an amino group and turn into thymine, creating a one-way mutational flow.
The map of mutational winds shows that the genome is nearly at equilibrium, but weak eddies remain in CpG valleys. These eddies help us understand how mutations arise and could be useful for studying cancer or comparing different species.
Chargaff's symmetry, which the model reproduces, was discovered by Erwin Chargaff in the 1950s and became a key to unraveling the structure of DNA by Watson and Crick.