the mouse fasl locus and known restriction sites were used to construct the targeting vector and diagnose homologous recombination in embryonic stem ( es ) cells and gene - targeted ( fasl , fasl ) mice . targeting knock - in vectors were made with the loxp / pgkneo / loxp cassette cloned into the pac1 site . targeting constructs for the mutant fasl mice ( supplementary fig . all experiments with mice were performed according to the guidelines of the animal ethics committees of our institutions . mice were killed at 6 , 12 or 20 weeks for analysis and further cohorts were monitored daily for morbidity and killed when showing signs of illness . tissues were fixed for microscopic analysis in 80% histochoice ( amresco)/20% ethanol or 10% buffered formalin and embedded in paraffin and conventional histopathology was performed on hematoxylin plus eosin stained sections . for detailed methods for immunohistochemical staining , immunofluorescent staining , confocal microscopy , cell preparation , flow cytometric analysis , elisa , aicd , chromatography , viral infection and target cell killing , t cell proliferation assays and western blotting refer to the online methods version . statistical analysis was performed using the student 's t - test , log rank ( mantel - cox ) test for survival curves or one - way analysis of variance using turkey 's comparison test to compare multiple groups where appropriate . the mouse fasl locus and known restriction sites were used to construct the targeting vector and diagnose homologous recombination in embryonic stem ( es ) cells and gene - targeted ( fasl , fasl ) mice . targeting knock - in vectors were made with the loxp / pgkneo / loxp cassette cloned into the pac1 site . targeting constructs for the mutant fasl mice ( supplementary fig . all experiments with mice were performed according to the guidelines of the animal ethics committees of our institutions . mice were killed at 6 , 12 or 20 weeks for analysis and further cohorts were monitored daily for morbidity and killed when showing signs of illness . tissues were fixed for microscopic analysis in 80% histochoice ( amresco)/20% ethanol or 10% buffered formalin and embedded in paraffin and conventional histopathology was performed on hematoxylin plus eosin stained sections . for detailed methods for immunohistochemical staining , immunofluorescent staining , confocal microscopy , cell preparation , flow cytometric analysis , elisa , aicd , chromatography , viral infection and target cell killing , t cell proliferation assays and western blotting refer to the online methods version . statistical analysis was performed using the student 's t - test , log rank ( mantel - cox ) test for survival curves or one - way analysis of variance using turkey 's comparison test to compare multiple groups where appropriate .