Fluorothymidine is an analog of the nucleoside thymidine (deoxythymidine) but the 3´-F atom prevents FLT from following the full biochemical pathway of thymidine. FLT is transported from the blood into cells by active transport; it does not freely diffuse across the cell membrane. Once in the cell, FLT is a substrate for thymidine kinase I (TK1) and is phosphorylated but is not incorporated into DNA. The assumption is that the concentration of FLT nucleotides in cells is proportional to TK1 activity and therefore to cellular proliferation. Phosphorylated FLT cannot exit the cell. FLT is not a substrate for thymidine phosphorylase and so is not significantly degraded in vivo and is retained in the cells. One advantage of FLT is that it is only a substrate for T and not for mitochondrial TK2 and so it is a more specific tracer compared with other fluorinated tracers for cellular proliferation.