To the Editor: The study by Sigurdsson et al. (April 12 issue)1 is an important one. There are, however, two factors that may confound interpretation of their data and its reproducibility. The first concerns tumor heterogeneity. Studies of other tissues (for example, a renal-cell carcinoma) by flow cytometry have shown that multiple samples may be needed to demonstrate aneuploidy.2 Furthermore, the degree of aneuploidy may vary within the tumor, and the biologic meaning of this is unclear.2,3 The second factor relates to the authors' failure to diagram how they established the S phase.4 Without an explanation, such an internal control,.