A method of screening for a protein involved in the extracellular regulation of latent TGF.beta. activation by transducing a cell line with a retroviral cDNA library to create a reporter cell line that produces green fluorescent protein (GFP) in response to TGF-.beta. signaling; growing individual clones created by the reporter cell line; co-culturing each individual clone with a second TGF-.beta. reporter cell line that produces luciferase in response to TGF-.beta., wherein the luciferase production identifies positive clones; and identifying a mechanism of latent TGF-.beta. activation that is employed by the positive clones. A TGF.beta. reporter cell line including a cell line; and a retroviral cDNA library, wherein the reporter cell line produces GFP in response to TGF-.beta. signaling. A method of screening for gene products involved in a biological process.