The telomere is a vital structure that protects our chromosomes. In a normal cell, telomere DNA shortens with each round of cell division—this gradual loss of telomere length eventually arrests cell growth. To gain immortality, cancer cells must turn-on a telomere maintenance pathway: the telomerase enzyme or ALT (Alternative Lengthening of Telomeres) pathway. ALT is used in 15% of cancers for telomere maintenance with a high prevalence in gliomas, pancreatic neuroendocrine tumours and sarcomas. ALT cancers show poor prognosis and response to therapy due to a lack of understanding of the pathway. The ALT pathway shows a set of cellular hallmarks. Among these hallmarks, the most commonly observed is the ALT associated body (APB), a morphologically enlarged form of the PML body. The APBs form de-novo around telomeres and assist with DNA homology-directed repair. I aim to identify the protein factors localized within both PML bodies and APBs in non-ALT and ALT cancer cells, respectively. To achieve this, I have fused PML (the major component of the PML-NB) to an APEX2 peroxidase and expressed it in a mouse embryonic stem cell (mESC), human HEK293 and ALT positive U2OS osteosarcoma lines. APEX2 acts as a promiscuous labelling enzyme by attaching biotin to proteins in close proximity (~20nm). Using these three lines, I have shown that the fusion protein is being stably expressed by western and immunofluorescence screening. The PML-APEX2 fusion protein will biotinylate the proteins contained within the PML bodies/APBs, which will be isolated using streptavidin beads for proteomic profiling and mass spectrometry analyses. The identification of factors that drive the formation of the APB will allow for a more targeted therapy towards ALT cancers.