question: Does Chromatin Immunoprecipitation (ChIP) show a bias for highly expressed loci?
answer: Several issues in the processing and analysis of ChIP-chip data have not been resolved fully, including the effect of background (mock control) subtraction and normalization within and across arrays. We detected a chromatin-state bias: open chromatin regions yielded higher coverage, which led to false positives if not corrected. The localization of unrelated proteins, including the entire silencing complex, to the most highly transcribed genes was highly suggestive of a technical issue with the immunoprecipitations.