Two conserved tryptophan residues of tumor necrosis factor and lymphotoxin are not involved in the biological activity.

Each of the two highly conserved tryptophan residues in hTNF (positions 28 and 114) was converted into phenylalanine by site-directed mutagenesis and the mutant proteins were partially purified. A cytotoxicity assay on mouse L929 cells showed only a slight reduction in biological activity, strongly suggesting that neither of the two amino acids is involved in the active site.