This is an Open Access article distributed under the terms of the Creative Commons Attribution License.-- et al. ; [Background]: An essential question in cancer is why individuals with the same disease have different clinical outcomes. Progress toward a more personalized medicine in cancer patients requires taking into account the underlying heterogeneity at different molecular levels. [Results]: Here, we present a model in which there are complex interactions at different cellular and systemic levels that account for the heterogeneity of susceptibility to and evolution of ERBB2-positive breast cancers. Our model is based on our analyses of a cohort of mice that are characterized by heterogeneous susceptibility to ERBB2-positive breast cancers. Our analysis reveals that there are similarities between ERBB2 tumors in humans and those of backcross mice at clinical, genomic, expression, and signaling levels. We also show that mice that have tumors with intrinsically high levels of active AKT and ERK are more resistant to tumor metastasis. Our findings suggest for the first time that a site-specific phosphorylation at the serine 473 residue of AKT1 modifies the capacity for tumors to disseminate. Finally, we present two predictive models that can explain the heterogeneous behavior of the disease in the mouse population when we consider simultaneously certain genetic markers, liver cell signaling and serum biomarkers that are identified before the onset of the disease. [Conclusions]: Considering simultaneously tumor pathophenotypes and several molecular levels, we show the heterogeneous behavior of ERBB2-positive breast cancer in terms of disease progression. This and similar studies should help to better understand disease variability in patient populations. ; JPL was partially supported by FEDER and MICINN (PLE2009-119), FIS (PI07/0057, PI10/00328, PIE14/00066), the Junta de Castilla y León (SAN673/SA26/08; SAN126/SA66/09, SA078A09, CSI034U13), the “Fundación Eugenio Rodríguez Pascual”, the Fundación Inbiomed (Instituto Oncológico Obra Social de la Caja Guipozcoa-San Sebastian, Kutxa), and the “Fundación Sandra Ibarra de Solidaridad frente al Cáncer”. AC was supported by MICINN (PLE2009-119). SCLL is funded by a JAEdoc Fellowship (CSIC)/FSE. MMSF and ABG are funded by fellowships from the Junta de Castilla y Leon. WR was supported by a Forschungsstipendium of the Deutsche Forschungsgemeinschaft (DFG) [RE 3108/1-1]. TN, BPB and DYL acknowledge support from the US Department of Energy Low-Dose SFA Program at Berkeley Lab [DE-AC02-05CH11231], the National Institutes of Health [RC1NS069177] and the California Breast Cancer Research Program [15IB-0063]. JHM was supported by the National Institutes of Health, a National Cancer Institute grant (R01 CA116481), and the Low-Dose Scientific Focus Area, Office of Biological and Environmental Research, US Department of Energy (DE-AC02-05CH11231). ; Peer Reviewed