Morphology of P-element insertions in polytene chromosomes. Possible scenarios: A - transgenic insertion into the interband results in formation of a novel band; B - electron microscopy image of the region 84E from chromosome arm 3R of wild-type (top) and transgenic for cHBΔ (bottom) larvae. Transgenic material forms a novel band (black arrow), which is absent from the chromosomes in control stock (white arrow); C - transgenic insertion does not result in formation of a novel band; D - electron microscopy image of the region 12E of chromosome X from wild-type (top) and cHBΔ transgenic (bottom) larvae. Chromosome morphology remains unaltered (black arrow) in the transgenic strain as compared to the wild-type chromosome (white arrow). Some marker bands are shown by arrowheads. Bar corresponds to 1 mkm.