into the dorsal flanks of SCID mice. Injected mice were examined weekly and tumor volumes were estimated from their length and width, as measured by calipers, using the formula tumor volume = l w260.52. Repeated Induction of EBV Reactivation The EBV-negative TW01 cell and EBV-positive NA cell were both engaged in this study to compare the effects between chemicals treatment alone and chemicallyinduced EBV reactivation. Repeated chemical exposures were achieved by incubating these cells periodically with 0.1 mg/ml MNNG, or TPA/SB, or TPA/SB in combination with MNNG. A group of mock treated cells was purchase Sodium laureth sulfate included as negative controls. The cells at the beginning of this experiment were defined as passage 0 cells. 24 h after seeding, the cells were subjected to a 24 h period of mock or chemical treatment. After this 24 hr treatment, the cells were recovered by replacing the medium with fresh medium and incubation for a further 48 h. The resulting cells were defined as passage 1. Cells from P1 were trypsinized and transferred and the process repeated again to obtain the P2 cells. “Pn”represents treated NPC cells, where n stands for the passage number of the cells. A total of ten treatments were applied to the cells in this study. For convenience, MG, TS and TS-MG are used for description of cells treated with MNNG, TPA/SB, and TPA/SB combined with MNNG, respectively. RNA Extraction and Semi-quantitative Real-time PCR RNA was extracted from NPC cells using the Qiagen RNeasy mini kit. The quality of total RNA samples was examined by Bioanalyzer 2100 to ensure the integrity of RNA samples. For quantification of genes of interest, the RNA samples were reverse transcribed into cDNA using the QuantiTect Reverse Transcription Kit. One twentieth of the cDNA was used in semi-quantitative real-time PCR for genes of interest using QuantiTect SYBR Green PCR kit. The calculations for determining the level of gene expression were made using the cycle threshold method. Utilizing the human GAPDH gene as the internal control, the relative quantitation values of a target template for each sample were expressed as 22DDCt, where DDCt = DCt NAP10/TS-MG 2DCt NA-P1/mock. The primer sequences used in this study 
are listed in Gene Expression Profiling The expression profile of the NA-P1/mock and the NA-P10/ TS-MG cells were analyzed by expression microarray. The analyses used whole-genome Affymetrix U133 Plus 2.0 human oligonucleotide microarrays containing over 47,000 transcripts and variants including 38,500 well-characterized human genes. Preparation of cRNA, hybridizations, washes, and detection were carried out according to the protocol recommended by the supplier. A duplicate of each sample was analyzed and the result data set was deposited in Western Blot and Immunofluorescence Assay The procedure for western blot assay of EBV lytic gene Rta, Zta and EAD, and immmunofluorescence assay of EAD in TPA/SB and MNNG treated NPC cells, are followed as describe previously. Synergism of Carcinogens Enhances NPC Progression the GEO database with accession number GSE38453. Public Domain Data A data set of expression profile of NPC, GSE12452, which was deposited by Paul Ahlquist in GEO, was included in the following data analysis. This data set includes expression profiles of 31 laser captured, microdissected NPC biopsies and 10 normal healthy nasopharyngeal tissue specimens, also assayed by Affymetrix U133 Plus 2.0 microarrays. Gene Expression Data Analysis altered after re