After years of extensive scientific discovery much has been learned about the 
networks that regulate epithelial homeostasis. Loss of expression or functional 
activity of cell adhesion and cell polarity proteins (including the PAR, crumbs 
(CRB) and scribble (SCRIB) complexes) is intricately related to advanced stages 
of tumour progression and invasiveness. But the key roles of these proteins in 
crosstalk with the Hippo and liver kinase B1 (LKB1)-AMPK pathways and in 
epithelial function and proliferation indicate that they may also be associated 
with the early stages of tumorigenesis. For example, deregulation of adhesion 
and polarity proteins can cause misoriented cell divisions and increased 
self-renewal of adult epithelial stem cells. In this Review, we highlight some 
advances in the understanding of how loss of epithelial cell polarity 
contributes to tumorigenesis.