Provided herein is a method for distinguishing an
aberrant methylation level for
DNA from a first
cell type, including steps of (a) providing a
test data set that includes (i)
methylation states for a plurality of sites from test
genomic DNA from at least one
test organism, and (ii) coverage at each of the sites for detection of the
methylation states; (b) providing
methylation states for the plurality of sites in reference
genomic DNA from one or more reference individual organisms, (c) determining, for each of the sites, the methylation difference between the test
genomic DNA and the reference genomic
DNA, thereby providing a normalized methylation difference for each site; and (d) weighting the normalized methylation difference for each site by the coverage at each of the sites, thereby determining an aggregate coverage-weighted normalized methylation difference
score. Also provided herein are sensitive methods for using genomic
DNA methylation levels to distinguish
cancer cells from normal cells and to classify different
cancer types according to their tissues of origin.