Strain differences in mouse embryonic fibroblast (MEF) proliferation. Strain differences in MEF proliferation were observed over 10 days and were well correlated with the observed strain differences in efficiency to generate induced pluripotent stem cells (iPSCs) during early reprograming. Non-transduced parent P3 MEFs from each strain were seeded at 1.9 × 105 cells per 60-mm tissue culture plate on day 0 and then counted every 2 days over a total of 10 days (A). MEFs were maintained in standard MEF media for the first 24 hours after which the media was changed to embryonic stem cell (ESC) media supplemented with doxycycline to simulate reprograming conditions. MEFs derived from two different embryos were evaluated for each strain (data presented as mean ± SD). The growth rate of the MEFs from each strain was determined from the slope of the linear regression curve fitted to the data set in (A) for each strain. The growth rate of each strain was then plotted against the total number of colonies positive for alkaline phosphatase (AP) staining and a line of best fit determined, revealing a moderately strong positive correlation between iPSC generation efficiency and MEF proliferation as indicated by the r2 value (B).