Persistent problems with false positive results were encountered when carrying out a published RT-PCR method to detect the CBFβ/MYH11 transcripts associated with the inv(16)(p13q22) cytogenetic abnormality in acute myeloid leukaemia. These were shown to be due to amplification of part of the intronic MYH11 sequence, presumably from very small amounts of contaminating DNA or unspliced primary RNA transcripts, amplified because of partial homology of the CBFβ3 primer to intronic MYH11 sequence.