Taken collectively, niclosamide causes amino acid starvation in cancer cells by at least three mechanisms: (i) inhibition of macropinocytosis that mediates the cellular uptake of proteins present in the tumor microenvironment, (ii) inhibition of amino acid transport via SLC38A5, and (iii) inhibition of SLC15A1 that mediates the cellular uptake of small peptides generated in the tumor microenvironment by the action of metalloproteinases on extracellular proteins such as collagen. The gene discussed is SLC15A1; the disease is cancer.