Adenylosuccinate lyase deficiency is an autosomal recessive defect of purine metabolism. Succinyladenosine (S-Ado) and succinylaminoimidazole carboxamide riboside (SAICAr) are the disease marker metabolites in physiological fluids. The Bratton-Marshall test for detection of SAICAr in urine has been added to the selective screening for inborn errors of metabolism that is carried out in our lab. During the last three years, around 2,000 patients have been screened by this method, resulting in the detection of four new cases with this disease. They all presented with severe psychomotor delay, hypotonia and refractory epilepsy since the neonatal period. The S-Ado/SAICAr ratio in cerebrospinal fluid was below 2, indicating that they correspond to the most severe form of the disease. New missense mutations were found in a heterozygous fashion in three patients. The study of purines in all patients with neurological disease of unknown etiology is highly recommended.

DAPI (4’-6-diamidino-2-phenylindole), a fluorochrome specific for AT-rich DNA, was supplied for 24 h at various concentrations to human leukocytes in culture. This treatment caused the appearance on the chromosomes of specific areas lacking spiralization. In particular, the centromeric regions of chromosomes 1, 9, and 16, a short region on the long arm of chromosomes 1 and 2, and the distal heterochromatic part of the long arm of the Y chromosome were despiralized. The despiralization pattern of DAPI is compared with those previously obtained with Hoechst 33258 and Distamycin A.

Cells of the Chinese hamster strain C-125 were treated for different time intervals with H 33258, a bibenzimidazole derivative. The same compound was used to stain fixed cells of the same strain. H 33258 induced in cells in culture specific areas of reduced spiralization on the metaphase chromosomes of some cells. These probably correspond to DNA segments rich in A-T bases interspersed along the chromosomes. Probably H 33258 acts during S period of cell cycle. The banding obtained by staining with H 33258 is similar to that induced by quinacrine dihydrochloride but shows a better resolution.

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The karyotype of the isopod crustacean Asellus aquaticus does not normally display any\ud heteromorphic sex chromosome pair. Some of the males in a wild population of A. aquaticus\ud collected in the Sarno river near Naples do display a heteromorphic chromosome pair. The heteromorphism\ud is due to the presence of two intercalary heterochromatic areas on one chromosome.\ud This chromosome is inherited through the male line as a normal Y chromosome. The heteromorphic\ud pair has retained the capacity to recombine during meiosis. In-situ hybridization of ribosomal\ud probes, labelled with digoxigenin—dUTP, reveals that ribosomal sequences are associated\ud with both intercalary heterochromatic areas of the heterochromosome. The ribosomal genes are\ud normally telomeric and associated with heterochromatin. After digestion of genomic DNA with\ud BamHI, EcoRl and HindIII restriction endonucleases and hybridization with ribosomal probes, the\ud hybridization patterns of the males with the heterochromosome differ from those of the males without\ud the heterochromosome, as well as from those of the females, which are identical. The possible\ud origin of the morphological differentiation of the heterochromosome, and the causes of the differences\ud in the ribosomal DNA restriction patterns linked to the presence of this chromosome, are discussed