Appearance of double minute chromosomes in somatic cell hybridisation experiments involving the hat selection system

In somatic cell hybridisation experiments using the HAT selection system, parental HPRT− cells have been shown to escape selection. These cells contain varying numbers of double minute chromosomes which are probably cytological consequences of amplification of the dihydrofolate reductase structural genes. When the double minute containing cells were challenged with varying concentrations of antifolate drug the number of double minutes per cell correlated well with the aminopterin concentration in the culture medium. The implications of these findings for cell fusion and gene transfer experiments is discussed.