There does not exist any information in the literature about the effect of time on PDX viability and growth, so we did not have any information to guide our estimation for the difference we expected to see between these groups. Therefore, we conservatively estimated there would be a 50% decrease in tumor formation at 48 hours as compared to Time 0. To detect a 50% decrease in tumor formation at a significance level of 0.05 and a power of 0.80, we calculated a sample size of 8 per group. In order to evaluate the effect of storage medium as well, we required a sample size of 8 at each time point for both the media and saline groups. For the UW-SCC34 and SCH772984 942183-80-4 UW-SCC52 experiments, all tumors were individually weighed at the end. The tumor weights obtained for these PDXs from the two growth mediums were summarized using descriptive statistics including mean and standard deviation. Tumor weights across the two growth mediums were compared using the Snedecor F-test from a twoway analysis of variance model that included time as a factor. Hence this model adjusted for the time when the sample was implanted into the mouse. The difference in the mean tumor weights between the media and saline growth medium samples was calculated with a 95% confidence interval. These statistical analyses were performed using SAS version 9.2. For the newly established PDX experiments, all tumors were harvested and weighed from the UW-SCC64 PDX. Mean tumor weights for the Time 0 and 24 hour groups were compared using a two-sample t-test with equal standard deviations. This analysis was carried out using Graphpad Prism v6.0d. For all statistical analyses a p-value less than 0.05 was considered statistically significant. As displayed in Table 2, the pre-implantation UW-SCC34 tumor was moderately differentiated with 10% keratinization, 5% necrosis/cystic change and an infiltrative pattern on H&E staining. Importantly, all tumors in the subsequent passage from the seven different time points and two storage mediums demonstrated the same characteristics of moderate differentiation with an infiltrative pattern along with some keratinization and necrosis/cystic change. Representative images from the stained slides are shown in Figure 4. Next, the UW-SCC52 pre-implantation tumor was evaluated and had moderate differentiation, no keratinization, 20% necrosis/cystic change and an infiltrative pattern. Once again, the histology of the tumors from the next passage was quite similar with each displaying moderate differentiation, no keratinization, some necrosis/cystic change and an infiltrative phenotype. Thus, histologically we have demonstrated that the same tumor developed regardless of the time or storage medium for both UW-SCC34 and UW-SCC52. PDXs represent an important and validated model for the investigation of numerous different cancer subtypes, especially in the trial of novel and standard chemotherapeutics. Because patient tissue is precious and often difficult to obtain, investigators must take great care to optimize this model system. For this reason we wanted to further explore the potential boundaries of PDX passaging by testing the hypotheses that 1) time from tumor excision to ultimate implantation in new NSG mice is important and 2) storage medium has an effect on PDX viability and growth. Surprisingly, we found that neither the storage medium nor time of storage affected PDX growth and establishment in a subsequent passage. Furthermore, we revealed that delaying the implantation of fresh patient tissue up to 24 hours did not affect initial PDX establishment. To fully appreciate the importance and necessity of PDXs in the field of cancer research, it is valuable to assess other available models along with their strengths and pitfalls. First, cell lines have played an important role in the history of oncologic research and drug discovery. HeLa cells represent the first human cancer cells grown in the laboratory, and analyses of these cells and other cell lines that followed have provided much of our current molecular understanding of cancer. Furthermore, the standard pre-clinical process for investigating novel cancer chemotherapeutics by the National Cancer Institute begins with assessment of in vitro activity in 60 established cancer cell lines followed by in vivo assessment of these cell lines in mice through both the hollow fibre assay and xenografts. Recently, the validity of utilizing cell lines as surrogates for the primary tumor has been brought into question. Gillet et al. demonstrated through genetic analyses that no correlation existed between patient tumors and established cancer cell lines, and in fact the cell lines bore greater resemblance to each other than to clinical samples. Additional groups have also revealed key differences between primary tumors and derived cell lines. However, investigators also note that important pathway changes still exist in these tumor cells. Thus, despite differences with respect to the primary tumor, genetic expression profiles can be employed to select cell lines for specific analyses. Due to the growing concern over the representativeness of cell lines to the primary cancer, there is the need for additional models to supplement this work. Transgenic mouse models represent another distinct system that have been utilized in cancer research. These models are most often used to assess the impact of relevant oncogenic mutations to tumorigenesis and therapeutic response for a variety of human cancers including pancreatic ductal adenocarcinoma, soft-tissue sarcoma, lung adenocarcinoma, HNC and many others. The scope of questions that can be examined by transgenic mice is quite broad as exemplified by prior studies on transgenic mice expressing human papillomavirus oncogenes in which interactions between these specific oncogenes with fanconi anemia deficiency genes, estrogen and cervical cancer progression, and oncoprotein expression in relation to lymphocyte trafficking have been elucidated. All in all, transgenic mouse research can provide unique insight into mechanisms of carcinogenic mutations and potential therapeutic interventions.