It was not realized at the time that nearly all attempts to establish human cell cultures resulted in cell lines with limited life spans. Dr. George Gey, the originator of HeLa, had sent his cells to all who requested them, and this problem arose because many workers were growing the immortal HeLa cell and mortal human cell strains in the same lab. Since the use of genetic markers to characterize and distinguish cell lines at the time was virtually non-existent, contamination by HeLa went undetected. In spite of the evidence, initially, the idea of laboratory errors leading to cross culture contamination was not universally accepted: an alternative explanation was that, when cultures became established, their G6PD phenotype changed. Gartler's original paper to Nature went to lengths to dismiss this possibility, surveying over 100 tumors to see if there was a phenotypic change in either G6PD or PGM, as well as trying other experimental approaches to test the idea. He concluded that "all evidence seems to point to the stability of the G6PD and PGM phenotypes both "in vivo" and "in vitro"." Further evidence against the possibility came when Nellie Auesperg and Gartler found a truly independently established human cell line, which demonstrated unique genetic markers. Cross culture contamination is now a generally accepted risk, and there are many genetic markers available to accurately characterize human cell cultures. However, the problem of cross culture contamination has not disappeared. Walter Nelson-Rees took up the issue some ten years after the original Gartler report, and continued to write about the problem for almost 25 years.