HIC peaks are labeled as in Fig. 1 B. Molecular masses are reported as neutral monoisotopic species. Predicted values were found using NCBI Protein database accession no. NP_005312 (H1.4) and NP_005310 (H1.2), assuming the loss of Met1 during protein maturation in vivo and residues 2–5 during electrospray ionization. Peak identifications are based on MS/MS sequencing of multiple electron capture dissociation fragment ions. The A142 and T142 allelic variants of H1.2 were detected initially by MS and confirmed by genotyping as described in Fig. S1. Phosphorylated residues were identified by MS/MS sequencing of electron capture dissociation fragment ions. Analyses of the H1.2 (T142) forms indicated that phosphorylated residues were identical to those determined for the H1.2 (A142) forms.