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Rhabdoid tumours are rare aggressive tumours of infancy. The definition classically relies on a characteristic morphology and the inactivation of the hSNF5/INI1 tumour suppressor gene. This entity includes central nervous system tumours (ATRT), renal tumours (RTK) and soft-part tumours. Their rarity and morphological pleomorphism make the diagnosis often challenging. However, the recently introduced immunohistochemistry with anti-INI1 (anti-SMARCB1) antibody is a very useful diagnostic tool. Deletions at the 22q11.2 locus and mutations in hSNF5/INI1 sequence must be investigated in order to confirm the diagnosis and to give insights on a presumable germline mutation. Indeed, a predisposition may be found in up to 30% of cases. The treatment is based on aggressive chemotherapy, surgery and irradiation. The prognosis remains poor and the survival rate is below 30%, whatever the anatomic location. Understanding the role of hSNF5/INI1 within the SWI-SNF complex for the epigenetic regulation of transcription might drive the future targeted therapies.

During the cell cycle, chromatin undergoes structural changes associated with cellular functions such as DNA replication during the S phase or the transcriptional inactivation during metaphase. All levels of chromatin organization from the nucleosome to the loops attached to the scaffold are concerned by these changes. Biochemical or biophysical studies from 70's, as well as more recent studies using nondestructive in situ methods, that allow to obtain information at the individual cell level, tend to demonstrate a correlation between the condensation state of the chromatin and the position of the cell in the cycle. The majority of these studies confirm the existence of one single cycle of condensation-decondensation for the chromatin. These studies are discussed in the first part of this chapter. In the second part, the role of the cdc (cell division cycle) gene's products in the control of the cell cycle and in the regulation of the chromatin structure are analyzed.

During dog-fish spermiogenesis, 2 basic nuclear protein transitions occur: the first from histones to spermatid-specific proteins S1 and S2, the second leading to protamines. S1, the most abundant transition protein, is a polypeptide containing 87 residues (Mr = 11,179 Da) whereas S2, the minor transition protein, contains 80 residues (Mr = 9,726 Da). The 2 proteins are mainly characterized by an asymmetry of the molecule, a very high content of basic residues, a relatively high level of hydrophobic residues and a cluster of acidic residues in the carboxy-terminal quarter of the molecule. The 2 proteins are phosphorylated on serine residues and the degree of phosphorylation is relatively important in protein S1. The 2 transition proteins are structurally unrelated to testis histones or sperm protamines and cannot be considered either as their proteolytic degradation products or as their precursors.

Gene expression is in great part under the control of proteins associated with DNA: histones which can be phosphorylated and acetylated, non-histone proteins which include enzymes, hormone receptors and other regulatory proteins. Phosphorylation seems to play an important role in chromatin regulation.

Several ultrastructural cytochemical methods are used to determine the constituents of the ripe nucleus of Equisetum arvense L. They show that: DNA, associated with an arginine-rich histone, is localized in central region of the nucleus; nucleoplasm is reduced to a thin peripheral coat and contains a probably lysine rich histone; RNA is not detectable; non histone proteins form lenticular amounts disposed against the nuclear membrane.