Apoptosis is a fundamental process by which multicellular organisms remove unwanted or dangerous cells and maintain tissue homeostasis. Murine A1 and the human homologue BFL-1 are pro-survival BCL-2 family proteins expressed almost exclusively in the haematopoietic system where apoptosis is crucial for the development, and maintenance of a healthy immune system. A1/BFL-1 has been implicated in many leukaemias and lymphomas as well as a small number of carcinomas. Furthermore, within these cancers high levels of A1/BFL1 have been correlated with treatment-refractory disease and poor patient outcomes1. In addition, it has been shown that enforced expression of a truncated version of BFL-1 in a xenograft mouse model of human leukaemia/lymphoma substantially accelerated tumour growth2. The c-terminus of A1/BFL-1 is important in its post-translational regulation by ubiquitylation, suggesting that the ubiquitin pathway of degradation constitutes an important tumour suppressing mechanism in the regulation of A1/BFL-13, 4. To date the ubiquitin E3-ligase(s) responsible for A1/BFL1 ubiquitylation remains unknown. Preliminary work by our collaborators identified the HectD1 E3 ligase as a potential candidate ubiquitinating A1/BFL-1. While HectD1 is not well studied, it is known that homozygous loss of HectD1 causes neural tube closure defects and exencephaly in mice5. My work is centralised around investigating whether HectD1 is a critical regulator of A1 protein stability and function. To answer this question I examined the consequences of loss of HectD1 on A1 stability in vitro and in vivo. The evidence I have collected suggests that loss of HectD1 alone does not affect A1 stability, but may regulate apoptosis and play a role in some thymic and peripheral T lymphocyte subsets and cells of the myeloid lineage.