RAB7B and fibrosarcoma: Interestingly, in HT1080 human fibrosarcoma cells, which have a constitutively high level of macropinocytosis similar to Dictyostelium, the transition from RAB5 to RAB7 appears to follow the association and fusion of RAB5-marked macropinosomes with RAB7-marked vesicles (Supplementary Fig. S12, Supplementary Movie 9), reminiscent of what occurs in Dictyostelium cells.