Two cell lines were derived from an embryonal rhabdomyosarcoma. Cells of both lines grew as monolayers in liquid medium and formed colonies in agar medium. Each line consisted of 2 cytologic types resembling those of the original tumor—spindle cells and large multinucleated cells. No myofibrils could be demonstrated in the cells by light or by electron microscopy nor were virus particles detected. Cells of both lines probably contained myosin‐ATPase and cells of line #2 contained myoglobin. Chromosome studies of cell line #1 revealed a stem‐line of 51 chromosomes with a consistent karyotype. Three passages of cell line #2 were studied, and no evidence of a stem‐line was noted. Chromosome counts ranged from 45 to 170, and consistent marker chromosomes in most cells were present in the form of large metacentrics in the C group.

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Lymphocytes from 20 individuals with Down's syndrome due to 13-15/21 centric-fusion translocations were studied by autoradiography after continuous late labeling with tritiated thymidine. In no case was chromosome 13 involved; chromosome 14 was involved in 18 cases, and chromosome 15 in two cases. These results are similar to those from 13 previously studied cases and indicate that the entry of chromosomes 13-15 into translocations is nonrandom. This nonrandomness is not a simple function of chromosome size or shape, since chromosomes 13-15 are acrocentrics of similar size.