Sentence:  Lastly, a "<WSD>toeprint</WSD>" of 30S subunits bound to T4 gene 32 mRNA has been observed with MMLV reverse transcriptase; while the experiment has not been quantitated, appearance of a signal at a subunit concentration of 0.02 μM is consistent with a binding constant on the order of 107 M-1 (Hartz et al., 1991). 
Meaning of the word marked with <WSD> tags:  A measure of 30S-mRNA binding.