Representational difference analysis (RDA) of human breast cancer was used to discover a novel amplicon located at chromosomal region 8q24.3. We examined a series of breast cancer samples harboring amplification of this region and determined that KCNK9 is the sole overexpressed gene within the amplification epicenter. KCNK9 encodes a potassium channel that is amplified from 3-fold to 10-fold in 10% of breast tumors and overexpressed from 5-fold to over 100-fold in 44% of breast tumors. Overexpression of KCNK9 in cell lines promotes tumor formation and confers resistance to both hypoxia and serum deprivation, suggesting that its amplification and overexpression plays a physiologically important role in human breast cancer.