In this case, we present our experience treating a patient with synchronous stage IAE non-Hodgkin low-grade B-cell lymphoma, of marginal zone origin, and stage IA (pT1a, pN0, cM0, G1, estrogen receptor (ER)+, progesterone receptor (PR)-, HER2-) ILC, both located within the right breast. This evidence concerns the gene ESR1 and B-cell non-Hodgkin lymphoma.