As expected, the numbers of these multinucleated cells do not change in the control experiments with HAb2 cells not treated with low pH, or treated with low pH without the trypsin pre-treatment that is required for transforming HA into a fusion-competent form (not shown).
In contrast, a short-term treatment of a low-pH medium to trypsin-treated HAb2 cells yields many multinucleated cells. These cells are formed by fusion, since they contain both of the cell trackers that initially are separated between different cell populations (Fig. 1)