An intrinsically disordered region, IDR^Ser271/272, is present in the FIT C-terminus and disrupted in the FITmSS271AA mutant (supports Figs. 2 , 3 , and 4 ). (A and B) Diagrams representing the PONDR scores for each amino acid position in A, FIT, and B, FITmSS271AA protein sequences. Analysis was performed via the tool PONDR-VLXT (Molecular Kinetics, Inc.). A score >0.5 indicates intrinsic disorder. The 0.5 threshold is marked with a red line. Above the graph, a schematic representation of the FIT protein showing the position of the bHLH domain in gray (126–201 aa) and subdivided into the basic region in blue (DNA binding site, 132–162 aa) and the helix-loop-helix region in black (dimerization site, 142–201 aa). Domain prediction was performed with InterPro (EMBL-EBI). FIT has four regions with a score >0.5 that are predicted IDRs, two of them in the C-terminal part following the bHLH domain, with one out of them comprising the position Ser271/272, indicated by an arrowhead, termed IDR^Ser271/272. In FITmSS271AA, the PONDR score dropped for this region below the threshold.