ATCC recommends limiting up to passage 5 (cancer cell lines) for cell culture studies in order to avoid significant genetic drift from reference cell line. Every time a cell culture is subcultured the passage number increase by one. When the cell population say at passage 4 is cryopreserved, the passage no is RESET back to zero! (If this cell is thawed and cultured it is designated as passage zero). I wonder if I have got this wrong. Something doesn't seem to be right. A cell line having repeatedly cultured and cryopreserved should have genetically drifted to another continent.