Selection of the appropriate CD45 conjugate is also important, since not all epitopes of CD45 are expressed on all WBC. Because of the differential splicing of the single CD45 pre-mRNA species, a variety of CD45 isoforms (polypeptides) can be synthesised and each one differentially glycosylated to produce a large number of unique glycoforms (34, 35). Restricted epitopes (CD45 R) are not expressed on all leukocytes. We have recently identified an apparent pan-CD45 reagent (IOL1b, Immunotech) that depends on sialic acids for its binding, and it too is variably expressed on leukocytes (unpublished observations). Thus, pan-CD45 antibodies that detect not just all isoforms but also all glycoforms of this structure are required.