Histone proteins are among the most highly conserved protein in eukaryotes. Histone H4 proteins from a pea and a cow, for example, differ in only 2 of 102 amino acids. However, comparison of the two gene (nucleotide) sequences shows many more differences. These observations indicate that mutations that change amino acids must be selected against. Required:
Why do you suppose that amino acid-altering mutations in histone genes are deleterious?

nucleosomal histones (H2A, H2B, H3 and H4) strongly interact among them and with DNA in order to form nucleosomes. In consequence, it is expected that mutations in histone genes have deleterious effects on the organization of the chromatin

Explanation:

Nucleosomes are well known to be the basic structural units of DNA packaging in eukaryotic cells. Each nucleosome consists of an octamer of histone proteins, i.e., two copies each of the histones H2A, H2B, H3, and H4, and one segment of DNA wrapped around this octamer of histones. Chromatin organization is fundamental o regulate key cellular processes, including gene expression and genome maintenance. In consequence, it is expected that histone proteins are evolutionary well conserved across the eukaryotic kingdom.