ESR1 and breast cancer: Analysis of HPRT1 expression in different subtypes of breast cancer showed that the basal subtype had the highest HPRT1 expression, followed by luminal B, luminal A, and HER2 (human epidermal growth factor receptor 2), respectively (Figure 2B), with levels being highest in estrogen receptor negative (ER-) tumors compared to estrogen receptor positive (ER+) tumors (Figure 2C).