SMAD4 and small cell lung carcinoma: A qRT-PCR analysis also showed that expression of TGFBR2 and SMAD3 was decreased in SCLC cells, but other TGF-β signaling components, including SMAD2, SMAD4 and TGFBR1 (the gene that encodes TβRI), were expressed at normal levels in these cells (Figure 1c).