MKI67 and fibrosarcoma: Figure 2A shows the HE staining of a ~100 mm3 HT1080 tumor, where the invasive tumor front presented finger-like structures penetrating muscle tissue. The tumors also presented a positive vimentin staining, characteristic of human fibrosarcoma and therefore useful to distinguish tumor from healthy tissue (Figure 2B). Additionally, CD31 (Figure 2C) and Ki-67 (Figure 2D) staining were performed to illustrate the highly angiogenic and fast-growing nature of this tumor model [22].