Cytotoxicity of peptides was measured by monitoring mitochondrial reductase activity using the TACS TM MTT cell proliferation assay (R&D Systems, Inc., Minneapolis, MN) according to the manufacturer's instructions. Dilutions of peptides in serum-free DMEM were added to confluent monolayers of LLC-MK2 cells in 96-well plates for 1 h at 37uC, similar to the focus forming inhibition assays, and incubated at 37uC with 5% (v/v) CO 2 for 24 h. Absorbance at 560 nm was measured using a Tecan GeniosPro plate reader (Tecan US, Durham, NC).