Direct karyotyping of tumour cells from three familial and two sporadic cases of retinoblastoma revealed the existence of a Dq- marker chromosome. The hypothesis is launched that a specific region on the long arm of one of the D chromosomes is the site of a locus which is essential for the sustained differentiation of specialized retinal tissue and may be the site of other loci essential for the maturation of other embryonic tissues. Fragility of this region and its potentiality for breakage under the influence of various environmental insults could be the basic cytological event leading to the development of sporadic retinoblastoma. Mutants at these loci, including those of sustained differentiation, could be a less common operational event whereby some variants could enhance the fragility of their respective chromosomal region and thereby explain the genetic transmission of retinoblastoma in certain families. It is common for the critical functional disruption of the locus to precede the deletion which may then be considered the terminal event in the fragile region.