MRC-5 (Medical Research Council cell strain 5) is a diploid cell culture line composed of fibroblasts, originally developed from the lung tissue of a 14-week-old aborted Caucasian male fetus. The cell line was isolated by J.P. Jacobs and colleagues in September 1966 from the seventh population doubling of the original strain, and MRC-5 cells themselves are known to reach senescence in around 45 population doublings.