Incorporating a radionuclide into a PARP inhibitor allows PET (11C, 18F) or SPECT (123I) imaging for in vivo visualisation of PARP expression, PARP inhibitor distribution, drug-target engagement and tumour uptake, thus providing important clinical information for diagnosis and staging, selection of patient subgroups suitable for PARP inhibition therapy, monitoring treatment response to genotoxic treatments or gauging the emergence of resistance. The gene discussed is PARP1; the disease is neoplasm.