TM4SF1 and mesothelioma: The general lack of knowledge surrounding the precise function of the four mesothelioma prognostic genes (ARHGDIA, COBLL1, PKM2, TM4SF1) indicated the importance of examining a normal cell line and WI38, while not ideal, was chosen because it is of similar embryonic origin (i.e., mesoderm) to mesothelial cells lining the thoracic pleura.