PARP1 and neoplasm: To date, pre-clinical anti-tumour studies combining PARP inhibition and radiation have been predominantly carried out in human subcutaneous xenograft models, which do not mimic many aspects of the tumour microenvironment that could affect responses to treatments.37 In addition, subcutaneous xenografts are not suitable for evaluation of normal tissue toxicity as generally only the skin is included in the field of radiation.