In this text about tau aggregation, what is K19

Question

Assembly of PHFs from tau protein (K19, 10 µM) will be performed at 37 °C in the presence of heparin (2.5 µM) in 50 mM NH4Ac, pH 6.8. Assembly will be followed quantitatively by fluorescence assay using thioflavin S. PHF formation from tau isoforms htau23 will be carried out in PBS buffer, pH 7.4, 10 µM protein, and 2.5 µM heparin. The samples will be incubated at 50 °C for 10 days.

Answer 1

Yes, of course it is especially K19.