Limiting dilution PCR was developed contemporaneously and
independently by our own group. It involved the conjunction of two
lines of research. In one line of research we had been using genetic
selection and lymphocyte cloning to study human somatic mutations
at the X-linked HPRT and autosomal HLA loci. Cell cloning
was used to amplify the rare mutant cells, Poisson statistics was
used to quantify their number and DNA sequencing was used to
analyse the nature of individual mutated genes. The second line of
research was our development of a PCR-based method to identify
and sequence the rearranged and mutated immunoglobulin heavy
chain (IGH) genes which can serve as clonal markers for neoplastic
lymphocyte clones in leukaemia. It was a natural step to merge
these two lines of research and to isolate and sequence the rearranged
IGH gene molecule at the time of diagnosis in a patient
with leukaemia, synthesise primers specific for the rearrangement,
and to use these primers and limiting dilution PCR to quantify the
marker IGH rearrangement and hence quantify the leukaemic cells
in samples obtained during treatment. A publication briefly mentioning
this method appeared in 1991 [6]. However, recognizing
the general utility of this method for quantification of DNA targets,
we published a definitive study of the method in 1992 [7] in a general
biological journal. We continued to use limiting dilution PCR
to study various aspects of treatment and biology of acute lymphoblastic
leukaemia (ALL). The most important study was a 1994
paper in the Lancet which showed that outcome in childhood ALL
could be predicted by the level of leukaemia after one month of
therapy [8], and treatment decisions based on this form of assessment
have now become part of routine management of childhood
ALL.