In this study, reverse-transcription polymerase chain reaction (RT-PCR) was used to examine histamine-N-methyltransferase (HNMT) expression in 50 randomly selected human breast cancer samples and to compare HNMT expression between 363 breast tumor and normal tissue pairs isolated by surgical and laser capture microscope dissections, Stable inhibition of HNMT protein expression was established in the SKBR3 breast cancer cell line using short interfering RNA (siRNA).