We also studied five cell line DNA samples from Coriell Cell Repositories (Camden, NJ). First, we measured their relative copy numbers using genomic DNA. The results showed that two of them have a single copy and two have two copies of the CYP2D6 gene per cell ( Table 2) . One sample had a relative copy number of about 1.17, equal to a diploid copy number of 2.34. We then STA-treated these five samples and ran the products on digital arrays. The relative copy numbers of the 1-and 2-copy samples remained the same and the fifth sample showed a relative copy number of about 1.5 or a diploid copy number of 3. Apparently this sample had a duplication of the CYP2D6 gene on one of the two chromosomes (35) . It has been previously demonstrated (31, 36) that when CYP2D6 duplication occurs, the two copies are separated by 12.1 kb. Therefore, the diploid copy number of 2.34 obtained when genomic DNA was used is likely the result of DNA breakage in this 12.1 kb genomic region in some DNA molecules that separated the two CYP2D6 copies. To confirm this, we ran a long range PCR [see (31) CYP2D6 duplication was observed only in the sample with a relative copy number of 1.5 ( Figure 4) .