This thesis focuses on a gene, lgl identified in a genetic interaction screen for novel negative regulators of G1 to S phase progression. The screen was carried out by isolating dominant suppressors of a Drosophila hypomorphic mutant of the key G1 to S phase regulator cyclin E, DmcycEJP. lgl (lethal giant larvae) is a tumour suppressor with a known role in epithelial and neuroblast cell polarity. This thesis''aim is to uncover the mechanisms by which the tumour suppressor Lgl functions to coordinate epithelial polarity and cell proliferation. Knowledge that will lead to the better understanding of the process of tumourigenesis.