It is frequently desirable to use normal genetically stable human cells as recipients of exogenous genes. However, normal human cell strains have not often been used because of their finite life-span. Instead, immortal human cell lines have usually been used as genetic recipients. These cell lines derived from tumors or by transformation with SV40 virus have accumulated numerous genetic changes (1 ,2 ), and may in fact contain changes that will affect the gene or genes that are of interest to a particular study. Normal human fibroblast cell strains have a proliferative potential of from 50–75 cumulative population doublings (CPD) (3 ,4 ). Since expansion from one cell to a million cells requires approx 20 population doublings, human fibroblasts can readily be used for stable transfection experiments. However, before initiating an experiment, the life-span of the recipient cell strains should be characterized, and electroporation should not be performed on cells with insufficient proliferative potential remaining.
