This presentation provides an overview of work for the National Institute for Occupational Safety and Health (NIOSH) to reevaluate dose and dose-rate effectiveness factors (DDREFs) for low-LET radiation used in the Interactive RadioEpidemiological Program (IREP). IREP is used to estimate probability of causation of radiogenic cancer. Radiation risk estimates used in IREP are obtained primarily from s1udies of Japanese atomic bomb survivors who received an acute dose of (mainly) low-LET radiation at a very high dose rate. In order to account for an assumption of reduced effectiveness of low-LET radiation in inducing cancer in humans at low doses and low dose rates, DDREFs are applied to cancer risk coefficients for all solid tumors that were derived from epidemiological data based on a, linear, nothreshold (LNT) model. Our main purpose is to provide NIOSH with alternatives to probability distributions of DDREFs currently used in IREP, based on a comprehensive evaluation of current information on low-dose extrapolation of radiation risks. Because the LNT model is now under challenge, a broad-based review and analysis was required that covered not only the relevant epidemiological, radiobiological, and microdosimetric information, but also emerging information on a diverse array of potential modifiers of the model, including adaptive responses, bystander effects, genomic instability, low-dose hyper-radiosensitivity, existence of a threshold, and hormetic responses. A critical piece of information is the revised set of dose-responses for cancer incidence in the atomic-bomb survivors, which is based on the new DS02 dosimetry system and additional follow-up of the cohort. This review will be used by NIOSH to determine whether the current version of IREP appropriately represents all sources of uncertainty in DDREF.