Chromosome fragile sites are a fascinating cytogenetic phenomenon now
widely implicated in a slew of human diseases ranging from neurological
disorders to cancer. Yet, the paths leading to these revelations were far from
direct, and the number of fragile sites that have been molecularly cloned with
known disease-associated genes remains modest. Moreover, as more fragile sites
were being discovered, research interests in some of the earliest discovered
fragile sites ebbed away, leaving a number of unsolved mysteries in chromosome
biology. In this review we attempt to recount some of the early discoveries of
fragile sites and highlight those phenomena that have eluded intense scrutiny
but remain extremely relevant in our understanding of the mechanisms of
chromosome fragility. We then survey the literature for disease association for
a comprehensive list of fragile sites. We also review recent studies addressing
the underlying cause of chromosome fragility while highlighting some ongoing
debates. We report an observed enrichment for R-loop forming sequences in
fragile site-associated genes than genomic average. Finally, we will leave the
reader with some lingering questions to provoke discussion and inspire further
scientific inquiries.