GPCR base confinement results in either exit or reentry. (A) Cilium kymograph of ^QDGPR161. Cells were treated with SAG for 40 min before the start of imaging. Green, red, and purple labels and line coloring indicate retrograde movements, confinement and anterograde transport (events i and ii), or exit from cilia (iii). (B) Time series of the three confinement events. A reference image of bulk NG fluorescence from ^APGPR161^NG3 was overlaid with images of ^QDGPR161. Time stamp (s) is in the top left corner. Bars, 2 µm. b, base; t, tip. (C) Centroid mapping of ^QDGPR161 during event iii. The ^QDGPR161 locations captured during the entire imaging session are shown as gray dots. The green track represents a processive retrograde transport event that precedes confinement of ^QDGPR161 (shown in red dots) at the base. Immediately before diffusion away from the cilium (purple track), ^QDGPR161 was nearly immobile for 4.5 s (purple dots). (D) Dot plot showing the base residence time of ^QDGPR161 during unproductive events (gray circles; n = 82) or before exit (green boxes; n = 3). IMCD3-[pCrys-^APGPR161^NG3] cells were treated with SAG for 40 min before imaging. (E) Scatter plot of areas explored by ^QDGPR161 during base residence events versus time. Gray circles represent unproductive events, and green circles represent preexit base residence events. Linear regression shows no obvious correlation between these two variables (Pearson correlation coefficient; r = 0.3). n = 17.