TOP2A and breast neoplasm: In the study of Hicks et al. [29] 50% of ERBB2-amplified breast tumors had TOP2A co-amplification and 16% had monoallelic deletion of TOP2A. In ERBB2 non-amplifed tumors, TOP2A was never amplified and in 5% of the tumors there were monoallelic deletions of both ERBB2 and TOP2A genes (ibid).