PROS1 and lung carcinoma: To this end, cell extracts from a p53-null H1299 lung carcinoma cell line that stably expressed a human p53 fragment [SFB-p53 (amino acids 100 to 355)] with a N-terminal triple-epitope tag, SFB (S protein, FLAG tag, and streptavidin-binding peptide) (Fig. 1A) were subjected to multistep affinity chromatography (streptavidin–agarose beads and S protein–agarose beads) as previously described (20).