The X chromosome region 9F12-10A7 (7 bands removed by Df(1)VL3) was saturated with lethal, semi-lethal, visible and male sterile mutations A total of 11 complementation groups were found. In the more narrow interval of Df()1)VL1 which removes 3 bands (10A1-2, 10A3, 10A4-5) 6 loci were localised. - The band 10A1-2 consists of a sereis of 5 different subunits: (i) silent DNA where no functions were found - at the distal edge of the band; (ii) and (iii) two genes: v and 1(1)BP4; (iv) silent DNA in middle of the band, (v) locus sev on the proximal edge of the band. About 70% of the band's DNA was found to be silent. - Using the set of chromosome rearrangements removing different parts of the band it was shown that these five sequences may function independently from each other.

The peculiarities of compact blocks appearing as a consequence of position effect variegation were studied in male polytene chromosomes. In T(1;2)dorvar7/Y males the frequency of nuclei with a block in the 2B region was lower at all temperature and the chromosome region involved in compaction was shorter than in T(1;2)dorvar7/FM6 females. The fraction of nuclei with blocks was considerably increased in dorvar7/0 males, especially at 18 degrees C when the viability of these males is sharply reduced. The following features distinguish the blocks in males from those in females: (i) compaction of the 2B region in the males results in genetic inactivation only to a very small extent; (ii) the structure of the blocks in males is diffuse; and (iii) the male blocks still maintain some transcriptional activity as indicated by 3H-uridine incorporation. The temperature-sensitive period of both block formation and genetic inactivation was found to be during the first 3 h of embryonic development.

This mini-review covers new data regarding the problem of the functional organization of polytene chromosomes: The localization of RNA synthesis in the polytene chromosome puffs, diffuse bands and interbands; The relative stability of banding pattern and its functional value; The informational content of bands.

Focuses of the l(1)BP4, vermilion and sevenless genes located in the same band 10A1-2 of Drosophila melanogaster X-chromosome, have been compared. For this purpose first the fate map of the blastoderm for 60 pairs of adult cuticular landmarks has been built based on scoring 612 gynandromorphs. The location of the legs and antenna primordia has been defined more exactly as compared to analogous maps by other authors. The location of new landmarks has been carried out: mesosternal bristle, sex-comb and first tergite primordia. The focus of the l(1)BP4 gene has been determined with the help of this map on the basis of analysis of 321 mosaics. The data obtained show that the lethal focus of the gene belongs to the "bilateral domineering" type and is located in the blastula area giving rise to the nervous system, behind the region of the third thoracic ganglia origin. Beside, there is a pair of autonomous nonlethal focuses of the same gene limited by the wings' zone. The focuses of the l(1)BP4 gene do not coincide with the known focuses of vermilion and sevenless genes, which means that all three genes are active in different tissues.