Our view of eukaryotic chromosomes is still very much dictated by the classic ideas of geneticists and cytologists considering the chromosome just as a vehicle for genes. This one-sided view of chromosomes may have been strongly influenced by the many cytological observations made on polytene chromosomes. They, in particular, revealed that the activation of genes for transcription is accompanied by local decondensation of the chromosome as it becomes evident in the formation of puffs in polytene chromosomes. The studies of chromosomes have for many years been focused on the relationship between genes and bands or interbands as they are seen in polytene chromosomes (or in the supposed equivalents, the chromomeres, of meiotic prophase chromosomes). This has yielded much fruitful informations such as the first direct evidence for the relationship between differential gene activity and cellular differentiation by the demonstration by Clever that steroid hormones directly regulate gene activity at the chromosomal level. Our view of eukaryotic chromosomes is still very much dictated by the classic ideas of geneticists and cytologists considering the chromosome just as a vehicle for genes. This one-sided view of chromosomes may have been strongly influenced by the many cytological observations made on polytene chromosomes. They, in particular, revealed that the activation of genes for transcription is accompanied by local decondensation of the chromosome as it becomes evident in the formation of puffs in polytene chromosomes. The studies of chromosomes have for many years been focused on the relationship between genes and bands or interbands as they are seen in polytene chromosomes (or in the supposed equivalents, the chromomeres, of meiotic prophase chromosomes). This has the very first directional evidence for the relationship between differential gene activity and cellular differentiation by the demonstration by Clever that steroid hormones directly regulate gene acti vity at the chromosomal level