Four different models illustrating the possible fate of IFT trains after they are returned to the base of the flagellum. (A,B) Closed model, (C) semi-open model, and (D) open model (see text for details). Large blue boxes, anterograde trains; small blue boxes, retrograde trains; and blue dots, IFT complexes particles. The orange, red, and green colors indicate the cytoplasmic, flagellum base, and flagellum compartment, respectively. IFT, intraflagellar transport.