Probes that were unmethylated in the 48 normal samples (mean β-value < 0.3) and that had a standard deviation (SD) of greater than 0.15 in the tumor samples were chosen for the clustering. In addition to β-values, we used M-values in this study (M-value = log2(β/1 β) because of the stronger signals for quantifying methylation levels (34). Unsupervised hierarchical clustering with Ward’s method and Euclidean distance measurement was used to cluster the 350 primary tumor samples based on methylation M-values, and the clustering dendrogram was cut at k = 2 to yield two clusters.
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