Statistics Data are presented as mean and standard deviation. Unless otherwise stated, statistical analyses were performed using unpaired Student’s t-test. Differences were considered statistically significant at p,0.05. The following symbols are used: *p,0.05; **p,0.01 and ***p,0.001. Supporting Information Figure S1 Tissue distribution of Sei1 and Sertad2 expression. (Top) Total RNA from a panel of mouse organs was isolated and analyzed by qRT-PCR for the expression of Sei1 relative to actin. (Middle) Sertad2 mRNA levels analyzed as before relative to actin. (Bottom) Ratio between Sei1 and Sertad2 expression levels. Found at: doi:10.1371/journal.pone.0008744.s001 (0.40 MB TIF) Figure S2 Survival curves of Sei1-null mice. Cohorts of 17 males and 16 females for the wt group, and 22 males and 23 females for the Sei1-null group, were aged until signs of terminal morbidity. Data are represented as Kaplan-Meier curves. Logrank test indicated that differences were not statistically significant. Found at: doi:10.1371/journal.pone.0008744.s002 (0.57 MB TIF) Figure S3 Cell cycle of Sei1-null cells. (A) Quantification of the experiment shown in main Fig. 2A (n = 2). (B) Quantification of the experiment shown in main Fig. 2C (n = 2). (C) Quantification of the experiment shown in main Fig. 2D (n = 2) Values correspond to the average and s.d. Student’s t-test was used to compare the two genotypes but no significant differences were found. References 1. Malumbres M, Barbacid M (2009) Cell cycle, CDKs and cancer: a changing paradigm. Nat Rev Cancer 9: 153–166. 2. Lowe SW, Cepero E, Evan G (2004) Intrinsic tumour suppression. Nature 432: 307–315. 3. Besson A, Dowdy SF, Roberts JM (2008) CDK inhibitors: cell cycle regulators and beyond. Dev Cell 14: 159–169. 4. Polager S, Ginsberg D (2008) E2F - at the crossroads of life and death. Trends Cell Biol 18: 528–535. 5. 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