Formed by the longitudinal data set of 446 breast cancer patients from Memorial Sloan Kettering Cancer Center assembled over a 25-year period, the Markov transition probabilities from site to site are estimated for each of the groups estrogen receptor (ER)+/human epidermal growth factor receptor 2 (HER2)−, ER−/HER2−, and HER2+. The gene discussed is ERBB2; the disease is cancer.