Primary liver cancer is the sixth most common cancer worldwide and the third highest cause of cancer related death. Mounting evidence suggests liver progenitor cells (LPCs) can act as a cancer stem cell, leading to the development of many liver cancers however key causal mechanisms involved in LPC transformation remain elusive. To identify these underlying changes, our lab has systematically assessed the expression levels of known oncogenes and tumour suppressors in a panel of LPC lines that have spontaneously transformed during routine culture. This system allows us to directly compare changes between the transformed cells and their non-transformed precursors. Using this approach we have identified the loss of Arf and Ink4a, two tumour suppressors encoded by the Cdkn2a locus, as a consistent change accompanying LPC transformation. Non-transformed LPC lines generated from embryonic and adult mouse livers readily express Arf and Ink4a; however, both are consistently not detected in the transformed LPC lines by Western blot or RT-PCR. Furthermore, PCR of genomic DNA indicates that the Cdkn2a locus is deleted in transformed LPC lines. To assess the role Arf plays in LPC transformation, Arf -/-embryonic LPC lines were derived from E14.5 embryos using the established “plate and wait” method. Interestingly, although the Arf -/- LPC lines retain the intact Ink4a gene, these cell did not express Ink4a. These cell lines were continuously passaged and regularly assayed in soft agar to assess their propensity to transform. Early passage cells (p8-9) did not form colonies in soft-agar, however, were present after another twelve passages suggesting the loss of Arf/Ink4a expression may contribute to the transformation of LPCs. Current studies are focused on providing definitive evidence that inactivation of the Cdkn2a locus directly causes LPC transformation leading to the development of liver cancer.