Molecular entities that localize in tumor tissue are clinically important for targeted delivery of diagnostic, imaging, and therapeutic reagents. Often these targeting entities are designed for specific receptors (e.g., EGFR or integrin receptors). However, there is a subset of cyanine-7 dyes that apparently localize in every type of solid tumor tissue (at least, no exceptions have been reported so far), and they persist there for several days. Consequently, these dyes can be used for near-IR optical imaging of tumors in animal studies, they can be conjugated with cytotoxic species to give experimental theranostics, and there is potential for expanding their use into the development of clinically useful derivatives. Data presented in the literature and in this work indicate that the half-lives of these compounds in serum at 37 °C is on the order of minutes to a few hours, so what accounts for the persistent fluorescence of these dyes in tumor tissue over periods of several days? Literature, solely based on tissue culture experiments featuring a particular receptor blocker, indicates that uptake of these dyes is mediated by the organic anion transporter proteins (OATPs). Data presented in this paper agrees with that conclusion for short-term uptake, but significantly expands understanding of the likely reasons for long-term uptake and persistent tumor localization in vivo.