To assess the in vitro effects of Erb-hcAb-RNase on tumour cell growth, the ErbB2-positive SKBR3 and the ErbB2-negative A431 cell lines were incubated with increasing concentrations of Erb-hcAb-RNase, Erb-hcAb or Trastuzumab, used as a control. This evidence concerns the gene ESR2 and neoplasm.