ERBB2 and melanoma: Specifically, the tumor-to-background ratios of the orthotopic HER2-positive tumors were significantly higher than those of the melanoma at all scan time points and consistently higher than those of the HER2-negative xenografts throughout the study duration, despite an increase in the ratios of the MDA-MB-468 xenografts being observed in the later scans (two-way ANOVA, p < 0.05) (Figure 4).