Apparent "in situ" clone of cytogenetically marked ataxia-telangiectasia lymphocytes.

Six adjacent metaphases, each with the same cytogenetic aberration of a group D chromosome, most probably a No. 14, were observed in a field of a slide from a 96-hour culture of lymphocytes from an individual with ataxia-telangiectasia (AT). None of the 304 other metaphases examined from this or other simultaneous cultures of this individual showed such an aberration. It seems most likely that an "in situ" marked clone has been observed and this supports interpretation of consistent cytogenetic abnormalities in those with AT as having clonal origin. The method of slide preparation employed which involves placing, rather than dropping, the cell suspension on the slide may facilitate detection of "in situ" clones.