Population doubling time: cells were plated in 6-well plates at a plating density of 1 × 105 cm2 in DMEM growth medium, supplemented with 10% FBS. Growth medium was renewed every 3 days. Cell counts were performed on the days indicated. Each point represents the mean of three different determinations made in triplicate. The population doubling times of the established cell lines (a) NIPBC-1 and (b) NIPBC-2 were calculated to be 33.25 hrs and 31.56 hrs respectively.