The percentage of n-mers that appeared exactly once (unique hits), out of all the n-mers detected in each genome. Slightly less than 50% of 16-mers detected in humans are unique. Whereas, for E. coli, a little more than 50% of 12-mers were unique.
Liu et al. BMC Genomics 2008 9:509 doi:10.1186/1471-2164-9-509