OK cells were treated with 1 μg/mL GTTR at 37°C or on ice and imaged live (insets) or fixed after specified time intervals. A1-A6) Cells at 37°C show increasing cytoplasmic GTTR binding over time. Insets show cells imaged live, and endocytotic uptake 15 minutes (or later) after GTTR application. B1-B6) Cells held over ice during treatment show increased GTTR binding over time, but less intensely than that seen at 37°C. Scale bar = 20 μm. Color gradient at base represents fluorescent intensity from the hot.lut lookup table: 0 (no fluorescence, black) to 255 (saturated pixels, white), and enhances the ability of the human eye to discriminate intensity differences over grayscale images. Note live images in insets were acquired without washing out GTTR from the extracellular medium, so fluorescence is visible outside the cells.