The 20F-3C eu-heterochromatin junction region of the In(1)w^m4h salivary gland polytene chromosome appears as a densely condensed chromosome block in the SuUR^ES mutant background.

Homozygotes show a reduced frequency of heterochromatization in the distal parts of the X chromosome of the T(1;2)dor^var7 rearrangement compared to animals which are wild-type for SuUR. Homozygotes show a reduced frequency of heterochromatization in the distal parts of the X chromosome of the Dp(1;1)pn2 rearrangement compared to animals which are wild-type for SuUR. XO SuUR^ES/+ males show a reduced frequency of heterochromatization in the distal parts of the X chromosome of the Dp(1;f)R rearrangement compared to animals which are wild-type for SuUR.

In the salivary gland polytene chromosomes of SuUR^ES mutants, a series of distinct bands not visible in wild-type appear in 80A-C, small bands in 81F become a large compact block and new, banded material is visible between chromosome 3 arms.

The pattern of completion of replication in the polytene chromosomes of SuUR^ES third instar larvae differs from that of Oregon R third instar larvae. The SuUR^ES larvae have a much lower number of late-replicating regions than Oregon R larvae. The sets of late-replicating sites in SuUR^ES and Oregon R are very similar on the whole. However, in many cases, regions that show typical late replication in Oregon R are not labelled in SuUR^ES animals. These regions include 21D, 22B, 22F 23A, 31A and many others. It appears that in SuUR^ES animals, the intercalary regions still complete DNA replication later in S phase than other euchromatic regions, but generally terminate replication earlier than in wild type.

A number of heterochromatic regions of polytene chromosomes show a reproducible banded structure in SuUR^ES mutants. The frequency of ectopic contacts between pericentric regions decreases in polytene chromosomes of SuUR^ES mutants.

In SuUR^ES mutants new bands appear in the 20BF region in polytene chromosome squashes. The banding pattern at the base of the chromosome 2 arms is more clear-cut in SuUR^ES mutants. A new well structured block of chromosome material named "Plato Atlantis", containing a total of 25 bands, is formed in the 80C3-81F1-2 interval in SuUR^ES mutant polytene chromosome squashes. The 101E region of chromosome 4 has no banding pattern and has the appearance of α-heterochromatin in SuUR^ES mutant polytene chromosome squashes. In the SuUR^ES mutant strain, there are no breaks in the 3C region, and the 3C2-3 and 3C4-5 bands appear as singlets. There is no break in the 7B1-2 band and it has the appearance of a large dense singlet. The 11A6-7 and 11A8-9 bands join to form a very large single band. Th 12E8-9 and 12F1-2 bands appear as singlets. The 19E1-4 region contains no break, the 19E1-4 block widens and three large single bands lying close to each other are occasionally detected within it. 33A1-2 appears as a large single band without evidence of breaks in the SuUR^ES mutant strain. 34A1-2 shows no features of a doublet in the SuUR^ES mutant strain and 36D1-2 becomes a single and very large band. The 39D1 and 39E1-2 bands are distinct and a dense chromatin complex is formed in the 39D2-5 interval. There is no break at 65B1-2 and the band appears large, dense and single. The 70C1-2 band is thicker in the SuUR^ES mutant strain than in wild type and appears as a singlet. The thickness of the 70D4-5 band is also considerably increased. The 71C1-2 band has no breaks in the SuUR^ES mutant strain and appears as a very large dense singlet. The 84D9-10 band is large in the SuUR^ES mutant strain without showing any signs of constriction. 86C1-3 and 86D1-5 show no evidence of weak spots in the SuUR^ES mutant strain.

Homozygotes do not show changes in developmental time.

SuUR^ES flies lack the "weak" points in the intercalary heterochromatin of the salivary gland chromosomes, lack ectopic contacts between chromosome regions and have additional intercalary heterochromatin in the centromeric regions. The intercalary heterochromatin region in 39E does not show DNA underreplication. The phenotype is semidominant.

The weak points found in wild-type larval polytene chromosomes are missing in all intercalary heterochromatin (IH) regions in homozygous larvae. The frequency of ectopic pairing between IH regions and between telomeres and pericentric heterochromatin (PH) is greatly decreased. Clear bands are seen in regions that form net-like β-heterochromatin in the basements of chromosome arms adjoining the chromocenters in wild-type animals. Additional material appears to be present in PH, for example between arms 3L and 3R a region as long as the whole of chromosome 4 is present which shows a typical banding pattern and is as thick as the euchromatic part of the chromosome, suggesting complete polytenisation of a normally underreplicated region of PH. Polytene chromosomes of heterozygotes have an intermediate phenotype between homozygotes and wild-type. The mutation also shows a maternal effect.