Relative Flourescence @ 60°C intrinsic substrate specificity requirement of the Endo IV enzyme which appears to require the rigid prolinol linker ( Figure 3B , n ¼ 0) used in the probes I-III. Second, the use of short probes (10mer to 12mer) gives the Endo IV a distinct advantage over most other assays where probes are typically much longer (1,2,6,25).