Human and mouse gene conservation is preserved at the protein level. Pairs of mAbs that recognize proteins encoded by human and mouse orthologs were used to stain human or mouse monocytes. Monocytes were analyzed using the gating strategy shown in Figure 1B. (A) Representative flow plots depict CD115 (M-CSF receptor/c-fms) surface expression on human and mouse monocytes. In human, these results were observed in at least 2 independent experiments and donors. CD115 was routinely used to identify mouse monocytes. (B) Representative flow plots and relevant isotype control staining of quantitative data depicted in panels C and D show differentially expressed human and mouse monocyte subset proteins. (C) Bar graphs portray the mean mean fluorescence intensity (MFI) above background (isotype control MFI was subtracted) and standard deviation of protein expression patterns that reinforce the analogy between CD16⁻ human and Ly-6C⁺ mouse monocytes or CD16⁺ human and Ly-6C^lo mouse monocytes. (D) Data show protein patterns of expression that are converse in relation to the proposed analogy between CD16⁻ human and Ly-6C⁺ mouse monocytes or CD16⁺ human and Ly-6C^lo mouse monocytes. (C-D) ■ represent CD16⁻ human or Ly-6G⁺ mouse monocyte subsets and □ represent CD16⁺ human or Ly-6G^lo mouse monocyte subsets. For proteins marked LE, some donors showed absence of expression, whereas others showed very low expression levels relative to isotype-matched control. Graphs show data from 4 to 12 human donors or 5 to 15 C57BL/6 mice. Quantification of mean fold change between the 2 subsets in each species is indicated below each graph. Unless marked with # (human CD81, Clec5a, CD9, mouse CD64), the difference in protein expression between the 2 subsets within each species is significant; P < .05 for human CD93 and mouse MHC II, and P < .02 for all remaining human and mouse proteins, Student t test.