Longitudinal imaging of human HSC and progenitor engraftment. Images are of representative mice that received transplants of luciferase-expressing human CD34⁺ (A) or CD34⁺CD38^- cells (B-C) and were imaged serially. All successive images in each row were taken from the same experimental mouse after transplantation at days shown. Exposure time was reduced for all animals at later time points as shown to avoid image saturation. Identical scales are used for transplants of CD34⁺ and CD34⁺CD38^- cells in panels A and B. In panel C, the same images shown in panel B are scaled to decrease signal intensity and more clearly demonstrate the anatomic distribution of engraftment.