End-point titration of sera. To show the values of end-point titration of sera from individual mice, we chose two groups of mice immunized three times with HPV16 L1 DNA either intramuscularly after cardiotoxin pre-treatment or by tattoo. Mice immunized intramuscularly with HPV 16 L1 DNA after cardiotoxin pretreatment developed lower levels of L1-specific antibodies than L1-tattooed mice. Serum values below the ELISA cut-off value of 0.4 optical density (O.D.) at 405 nm were considered to be negative.