He is internationally recognised for research on the enzyme telomerase and for discovering ALT (Alternative Lengthening of Telomeres), which together contribute to unlimited growth in >95% of cancers. His team has made additional seminal findings regarding ALT and the manner in which it is repressed in normal (non-cancer) cells, and identified a molecular marker for ALT. Overall his research aims to help develop anti-cancer treatments that would work by blocking immortalisation, and tests for detecting signs of cellular immortalisation for early detection of cancer. Professor Reddel has won numerous awards including the Ramaciotti Medal for Excellence in Biomedical Research (2007), the 2011 NSW Premier's Award for Outstanding Cancer Researcher of the Year, and the Neil Hamilton Fairley Medal of the Royal Australasian College of Physicians (2017). He was a founding member of the University of Sydney's Cancer Research Network and chaired its Translational Research Committee. He is a member of the Board of Luminesce Alliance, which has been set up to coordinate paediatric research in NSW.

Professor Reddel’s research on the molecular genetics of cancer cell immortalisation focuses on the processes by which telomeres in cancer cells remain lengthened thereby facilitating evasion of senescence, i.e. studies on the enzyme telomerase and an alternative mechanism of lengthening of telomeres (ALT). The latter field of research was created after his team’s discovery of ALT in human cell lines, tumour-derived cell lines and in tumours. His team has since made major findings including: the discovery of a novel type of PML body in ALT cells that enables diagnosis of ALT by immunostaining of routine tumour sections; demonstration of ALT activation due to loss of repressor gene(s); cloning of the human telomerase catalytic subunit, hTERT (with an Australian consortium led by Dr Andrzej Kilian, Canberra); first definitive identification of the subunits of active human telomerase enzyme; showing that the presence of ALT activity is a strong predictor of outcome for high-grade brain tumours; and discovery of a molecular marker that is rapidly responsive to changes in ALT activity.