A class of near infrared fluorescence (NIRF) heptamethine cyanine dyes, in their native form without chemical conjugation, can be taken up and accumulated specifically in cancer cells and used for tumors imaging. The different uptake of the dyes into cancer but not normal cells is possible due to the presence of specific isoforms of organic anion-transporting peptides (OATPs), and regulated by hypoxia. These advances have widely extended current application of NIRF imaging in cancer diagnosis.