An abnormal CAG repeat expansion occurring in the first exon of the huntingtin gene (HTTex1) is the cause of Huntington’s disease (HD). In the N-terminal region, the huntingtin protein (HTT) contains 3 phosphorylatable residues - threonine 3 (T3) and serines 13 and 16 (S13/S16) - that, upon phosphorylation, strongly modulate aggregation and toxicity of mutant HTTex1 fragments. Here, we present evidence that phosphorylation of T3 prevents the diphosphorylation of S13/S16, while its dephosphorylation increases the chances of S13/S16 diphosphorylation in mutant HTTex1-expressing cells. This single observation has massive potential relevance for the understanding of HTTex1 structure and function and the treatment of HD.