The National Comprehensive Cancer Network (NCCN) panel accepts the updated 2016 version of the ASTRO APBI guideline, which now defines patients “suitable” for APBI to be the following: 1) 50 years or older with invasive ductal carcinoma (IDCA) measuring ≤2 cm (T1 disease) with negative margin widths of ≥2 mm, no lymphvascular invasion, estrogen receptor (ER) positive, and BRCA 1/2 negative or 2) screening-detected ductal carcinoma in situ (DCIS) with low/intermediate nuclear grade, and tumor size measuring ≤2.5 cm with negative margin widths of ≥3 mm (1). The gene discussed is ESR1; the disease is invasive ductal breast carcinoma.