The role of UVB-induced apoptosis in the formation of squamous-cell carcinoma (SCC) is recognized. We previously identified the small RhoB GTPase, an early response gene to cellular stress, as a critical protein controlling apoptosis of human keratinocytes after UVB exposure. Here we generate SKH1 mice invalidated for RhoB to evaluate its role in UVB-induced skin carcinogenesis in vivo. We show that rhob-/- mice have a lower risk of developing UVB-induced keratotic tumors and actinic keratosis which is associated with a higher sensitivity of UVB-exposed keratinocytes to apoptosis. We extend this observation to primary cultures of normal human keratinocytes in which RhoB was down regulated with siRNA and further show that the RhoB-mediated hypersensitivity to apoptosis depends on Bcl-2 down regulation. In rhob-/- mice, the UVB-induced tumors were preferentially undifferentiated and highly proliferative. Lastly, we show in human an almost constant loss of RhoB expression in undifferentiated SCC. These undifferentiated and RhoB-deficient tumors have elevated γH2AX and 53BP1, two markers of DNA double-strand breaks. Together, our results indicate that UVB-induced RhoB expression participates to in vivo SCC initiation by increasing keratinocyte survival. Conversely RhoB may limit tumor aggressiveness as loss of RhoB expression in tumor cells is associated with tumor progression.Journal of Investigative Dermatology accepted article preview online, 21 June 2013; doi:10.1038/jid.2013.278.