Onfekafusp alfa (also known as L19TNF) is a homotrimeric fusion protein consisting of the L19 single-chain antibody fragment (scFv) fused to human tumor necrosis factor (TNF). The L19 scFv specifically targets the extra domain B (ED-B) of fibronectin, a protein that is abundantly and selectively expressed in the tumor vasculature and stroma of many solid tumors but is virtually absent in normal adult tissues. This targeting directs the potent cytokine TNF to the tumor microenvironment. Once localized, TNF exerts its pro-inflammatory and cytotoxic effects on tumor vessels and stromal cells, leading to vascular disruption, immune cell recruitment, and tumor necrosis. It is being developed as a targeted immunocytokine for the treatment of advanced solid tumors.