Strain specific CTLA4 and 4-1BB expression analysis in homozygous B-hCTLA4/h4-1BB mice by flow cytometry. Splenocytes were collected from WT and homozygous B-hCTLA4/h4-1BB (H/H) mice stimulated with anti-CD3ε in vivo and analyzed by flow cytometry using species-specific anti-hCTLA4 and anti-h4-1BB antibodies. Mouse CTLA4 and 4-1BB were detectable in WT mice but not in homozygous B-hCTLA4/h4-1BB mice. Human CTLA4 and 4-1BB were exclusively detectable in homozygous B-hCTLA4/h4-1BB mice but not in WT mice.