T22-GFP-H6-FdU chemical synthesis: T22-GFP-H6 is first covalently bound to the 6-Maleimidohexanoic acid N-hydroxysuccinimide ester linker through its amino groups in the external lysines (Hermanson et al, 2013). The thiol-functionalized oligo-FdU (oligo-(FdU) 5-SH) (see Appendix Fig S1) is then reacted with T22-GFP-H6 functionalized with maleimide (Michael reaction). 