FITC and FAM are both derivatives of fluorescein. FITC, known as Fluorescein 5(6)-isothiocyanate, is a mix of isomers, which has a isothiocyanate reactive group (-N=C=S) at one of two hydrogen atoms on the bottom ring of the structure. 6-FAM also known as 6-carboxyfluorescein is a single isomer. FITC and 6-FAM have identical spectral characteristics. In experiments, 6-FAM performs exactly like FITC. But 6-FAM is often used for oligo labeling since it can be incorporate at time of synthesis or conjugate post-synthetically. The chemistry of 6-fAM is more robust and result in better yield. 6-FAM is less susceptible to photobleaching.