Use of third-color tracking oligonucleotide for quality control and data filtering. Pre-hybridization fluorescein image (A) and post-hybridization Cy3/Cy5 image (B) of M. tuberculosis array possessing the tracking oligonucleotide. Open squares; denote elements where oligonucleotides were not deposited on slide surface. Open circles; denote elements where target-specific M. tuberculosis oligonucleotides were present in low concentration. Black arrowheads, negative controls containing tracking oligonucleotide only.