The patient RNA was compared to the normal RNA pool using a T7-based amplification method: 1 microgram of total RNA was reverse-transcribed using a T7-oligodT primer and then labelled aRNAs produced via in vitro transcription in the presence of either Cy3-UTP or Cy5-UTP. The labelled aRNAs were then purified and hybridized competitively to the microarray.