preliminary data demonstrating bioavailability (if applicable) and documentation that the natural product produces a reproducible and measurable impact on target engagement (i.e., measure of the mechanism of action). Only in cases when it is not possible/practical to measure target engagement in the patient population of interest or when there is a fundamental understanding of the products mechanism of action will this preliminary data requirement be waived. Applications submitted to this NOFO should propose a mid-phase clinical trial to do the following: determine the optimal dose or formulation of a given natural product for use in a future multi-site efficacy trial; or determine which patient phenotypes will be responders versus non-responders to the natural product to inform inclusion/exclusion criteria of a future efficacy trial. Clinical trials submitted under this NOFO are expected to be hypothesis based, milestone-driven, and directly related to the research priorities and mission of NCCIH. This NOFO will not support single-site or multi-site efficacy or effectiveness trials, nor will it support trials to test natural products for the treatment or prevention of cancer. Applicants are encouraged to contact the appropriate NCCIH Scientific/Research contact for the area of science for which they are planning to develop an application prior to submitting to this NO