Because q(x) takes integer values, R(x) takes discrete values. The smallest possible value is [2(1 - α)/D], which occurs at homozygously deleted loci and corresponds to the fraction of DNA from normal cells. The spacing between values [α/D] corresponds to the concentration ratio of alleles present at one copy per cancer cell and 0 copies per normal cell. Importantly, if a cancer sample is not strictly clonal, copy-number alterations occurring in substantial subclonal fractions will appear as outliers from this pattern (Fig. 1, Supplementary Fig. 1a-c, arrows).
