It was perhaps a lucky coincidence that the early attempts to establish the chemical composition of the cell nucleus were initially carried out on such diverse biological systems as salmon sperm heads (1) and geese and chicken erythrocytes (2). Examination of sperm and erythrocyte systems, respectively, lead to the protamine and histone concepts (3). We know with certainty that, with the exception of the male gametes, all somatic cells exclusively contain histones. Hence, in metazoans, protamines (4, 5) are confined to the sperm nuclear chromatin, and even among sperm, protamines are not always present. Indeed, a large number of metazoans contain somatic-like histones in their sperm, and some crustaceans (order Decapoda) lack any chromosomal proteins in their sperm (6, 7). Therefore, in contrast to the somatic nucleus, sperm chromatin may have a much more diverse protein composition. It was not until the first attempt of classification of the sperm nuclear basic proteins (SNBPs) 2 by David Bloch (6,8), an effort later on extended by Harold Kasinsky (7), that a clearer picture started to emerge in this regard.More recently, an enormous effort has been carried out in several laboratories, including our own (9 -15), to extend this analysis to a large number of representative organisms from the different phylogenetic groups. With a broader perspective now available, SNBP heterogeneity can be restricted to three major groups or types: histone (H), protamine (P), and protamine-like (PL) (16).Histones consist of core histones (histones H2A, H2B, H3, and H4) and linker histones (histone H1 family). The names refer to the structural role of these proteins. Core histones are responsible for constraining DNA wrapped about a histone core to produce a nucleoprotein complex (chromatin subunit) known as a nucleosome core particle. Linker histones bind to the linker DNA regions connecting adjacent nucleosome core particles and assist in the folding of the chromatin fiber (17).Protamines are a highly compositionally and structurally heterogeneous group of proteins (9). They exhibit a high charge density and a prevalence of arginine in their composition (13), a fact that it is most likely related to the higher affinity with which this basic amino acid binds to DNA (18). They lack any secondary structure in solution but may adopt a folded conformation upon interaction with DNA. Protamine-like proteins share compositional and structural similarities between histones and protamines (9, 16). Hence they represent a structurally intermediate group that will be discussed more extensively in the following sections of this review.To a certain extent, all three types of SNBP can be considered structurally analogous as all of them produce folded chromatin fibers of 30 -50 nm (19) regardless of the particular structure of the individual nucleoprotamine complexes. At the functional level, somatic histones bind to DNA in a highly dynamic way that not only helps in the folding of the genome but also has an important role in the epigenetic reg...