ResearchSubject is a linking resource and cannot be itself a Participant. This use case would always handled by identifying a Patient resource (which may be a de-identified resource for the purposes of research participation), rather than trying to use ResearchSubject as the participant directly. However, it is reasonable to add other allowable potential participants including: Medication Specimen - Note some clarifying language needed here to ensure appropriate use (e.g. as a specimen in cell-line research is acceptable, but specimen in clinical research would typically be represented as the focus rather the subject) Substance BiologicallyDerivedProduct