Use of EGFRt as a marker for in vivo detection of engineered cells. (A) Day 20 bone marrow cells harvested from mice that were engrafted with either unmodified huEGFRt-negative control T cells (Ctrl T cells) or huEGFRt-selected T cells (EGFRt⁺ T cells) were stained using peridinin chlorophyll protein-conjugated anti–human CD45 and biotinylated cetuximab followed by PE-conjugated streptavidin. Total percentage of CD45⁺ cells in bone marrow are indicated in the left-hand panels. Percentage of EGFRt⁺ cells within the CD45-gated population are indicated in the right-hand panels, using quadrants that were created based on isotype control staining. (B) Day 20 femurs were also analyzed by immunohistochemistry for CD45 versus EGFR expression. EGFR expression on paraffin-embedded breast tumor tissue was used as a positive control for EGFR staining. Representative ×40 and ×100 images are shown as indicated.