palladium catalyst the fluorophore was attached to the base via a photocleavable linker that could easily be cleaved by a 30 second exposure to long wavelength uv light thus either disulfide reduction or photocleavage can be used as a cleavable linker another approach to reversible termination is the use of natural termination that ensues after placement of a bulky dye on a dntp the presence of a charged bulky dye on the dntp can act as an effective terminator through steric and or electrostatic hindrance the presence of one incorporation event prevents further incorporations unless the dye is removed cleavage of the dye removes the fluor and effectively reverses the termination examples of modified nucleotides are also described in u s pat no 7 427 673 and u s pat no 7 057 026 the disclosures of which are incorporated herein by reference in their entireties additional exemplary sbs systems and methods which can be utilized with the methods and systems described herein are described in u s patent application publication no 2007 0166705 u s patent application publication no 2006 0188901 u s pat no 7 057 026 u s patent application publication no 2006 0240439 u s patent application publication no 2006 0281109 pct publication no wo 05 065814 u s patent application publication no 2005 0100900 pct publication no wo 06 064199 pct publication no wo 07 010 251 u s patent application publication no 2012 0270305 and u.