The mammalian central nervous system is remarkable for its
high degree of organization among vastly heterogeneous cell
types of varied function. The molecular basis for this heterogeneity
is thought to involve complex regulation, at both transcriptional
(1) and posttranscriptional levels (2), of large and diverse
gene families (3, 4). These and virtually all other mechanisms
implicated in brain development, function, and disease are
assumed to operate on a constant genome.
Indirect evidence for some formof somatic genomic alteration
has come from nervous system expression and neurogenetic
effects of a growing list of molecules that function in DNA
recombinationrepair and surveillance (5–14). Interestingly,
many of these genes are also implicated in cancer, where a
commonly associated sequela is aneuploidy. Precedent exists for
aneuploidy during early mammalian development (15, 16) where
it is thought to result in cell death. These observations led us to
ask whether the number of chromosomes in neuroblasts and
neurons is variable.