Identification of the two variants of histone H4. A. MALDI-TOF mass spectrum showing that the amount of the peptide with calculated mass of m/z 1003.6 from histone H4.1 was much more than that of the peptide (m/z 989.6) from histone H4.2 in the peptide mass fingerprinting of trypsin digested histone H4. B. MS/MS spectrum showing peptide (m/z 1003.6) corresponding to 60IFLENVIR67 of histone variant H4.1. C. MALDI-TOF mass spectrum showing the peptide (m/z 989.6) from histone H4.2 after nano-LC separation. D. MS/MS spectrum showing the peptide (m/z 989.6) corresponding to 60VFLENVIR67 of histone variant H4.2.