CD5 and small cell lung carcinoma: Several molecules have been implicated in the regulation of tonic TCR signaling, among which CD5 and Nur77 are two of the most extensively characterized and functionally relevant markers.[28, 29, 30] To elucidate the key regulatory factors governing tonic TCR signaling within the SCLC microenvironment, we performed targeted knockdown of CD5 and Nur77 in naïve CD8+ T cells.