While invasion data of the M13HS-2 and -8 tumor hybrids suited well to the assumption, that tumor hybrids often exhibit novel properties, such as an enhanced metastatic capacity [30,38,39,73,74], it remains unclear why the invasive capacity of M13HS-2 ZEB1-KO cells was not diminished, but rather comparable to M13HS-2 wildtype cells. This evidence concerns the gene ZEB1 and neoplasm.