We present the case of a Caucasian man with rheumatoid arthritis (RA) who was being treated with adalimumab (Humira®, Abbott Laboratories, Global Health Care and Medical Research, 100 Abbott Park Road, Abbott Park, Illinois, USA), an anti-tumor necrosis factor α (TNFα) biological agent, who developed synchronous B-cell lymphoma and locally metastatic breast cancer. This evidence concerns the gene TNF and B-cell non-Hodgkin lymphoma.