EGFR and neoplasm: One major shortcoming of the above-mentioned methods is that although GS-cells resemble the genetic and transcriptional phenotype of the original tumor closely [13, 14], EGFR amplification as the most frequent molecular alteration is usually not preserved in vitro [49]. EGFR amplification can only be maintained for a limited number of passages in vitro (n < 5) either using conventional or GS cell culture conditions at normoxia (21% O2) or at hypoxia (1% O2) [14, 69].