Importantly, the use of pre-tolerized GH mice minimizes or eliminates these aberrations, resulting in more reliable, tractable preclinical models. For preclinical studies, a cryogenically preserved labeled tumor was revived and expanded by subcutaneous transplantation into mice. These tumors were resected upon reaching 500 mm3 and expanded through passage into the requisite number of mice for the actual studies described in the text. For preclinical models of spontaneous metastasis, primary tumors were surgically removed upon reaching 500 mm3, and the mice were randomized into groups according to the study design. Hygromycin B metastasis and recurrence were monitored periodically by imaging using the Xenogen IVIS system to measure BL flux. The control group received vehicle solution, and the experimental group received treatments of chemotherapeutic agents. The dose and schedule in each experiment have been specified in the Results. When mice showed signs of morbidity, defined by the animal study protocol, they reached their endpoint and were euthanized for further study. These results indicate that immunity against xenobiotic reporters can suppress the metastatic potential of Apoptosis Activator 2 transplanted labeled cancer cells, and highlight the advantages provided by the GH mouse for monitoring cancer progression and cell tracking. We corroborated and expanded our assessment of the GH mouse using ffLuc-eGFP-expressing LLC cells. Well-labeled LLC cells were transplanted subcutaneously into GH, WT and also NOD/SCID mice, which have residual innate immune activity, and arising tumors resected at the same size. In the first imaging after resection, metastases arose with higher BL levels in GH mice relative to those in WT and NOD/SCID mice. These results demonstrate that immune responses against xenobiotic reporters can restrict the growth and metastatic potential of labeled tumors in immunocompetent and even partly immunocompromised mice, a problem that could be overcome through the use of GH host mice. Previously, we demonstrated the feasibility of tracking cancer recurrence and progression with BL imaging in metastatic models.