Amino acid transporters in the plasma membrane are essential to supply cells with amino acids for cellular nutrition. For continuous growth and proliferation, cancer cells require more supply of amino acids so that they upregulate amino acid transporters to compensate massive amino acid requirement. Among amino acid transporters in cancer cells, LAT1 (L-type amino acid transporter 1, SLC7A5) is particularly important, because it is responsible for the uptake of essential amino acids by cancer cells. LAT1-specific PET probe, 3-fluoro-L-α-methyl-tyrosine (FAMT), has demonstrated that LAT1 is highly specific to cancers. It is remarkable that FAMT can differentiate cancers from non-cancer lesions including inflammation. LAT1 is, thus, considered to be both a diagnosis marker and a therapeutic target of cancers. By means of structure-activity relationship analysis of the ligands and chemical synthesis of series of compounds for structure expansion, we have developed LAT1-specific high-affinity inhibitors. They down-regulate mTORC1-mediated signaling pathway and suppressed the growth of Xenograft tumors in mice. They are now in the process of clinical trials for anti-tumor drugs. The combination of LAT1-selective PET probes and LAT1-specific anti-tumor agents would provide us with effective cancer treatment by targeting the same molecule for cancer diagnosis and therapeutics.