: Why do gene therapies so often target rare disease? I read on {1}: At the time of writing, clinicaltrials.gov has well over 100 open gene therapy clinical trials and many of these are for rare diseases. Why do gene

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Common diseases tend to be polyfactorial, while rare genetic diseases often involve a single gene.?Many involve just a single mutation event (substitution, deletion, addition, inversion, etc).

It's easier to target a single-gene defect than multiple.?Since gene therapies are at such an early stage of development, it's expected they would go for low-hanging fruit.

Go to clinicaltrials.gov and search for gene therapy.?Consider the genetic interventions (skip the drug trials).?The majority on the first dozen pages specify a single target gene.

I was unable to find a list of non-infectious diseases ranked by prevalence.?However, there is a List of Common Diseases on Ranker.?It is suitable for illustrative purposes.?Scrolling through the index of a textbook would do as well.?Skipping psychiatric, infectious, and situational conditions (such as traumatic brain injury), consider whether there is an obvious genetic target.?There isn't one for conditions such as Dementia, Diabetes Mellitus, Cancer, Stroke, etc.

Also consider CDC: Leading Causes of Death.?They're largely non-genetic.

DM-II is highly heritable, so in that sense, it is genetic.?But usually, people consider only single-gene conditions to be "genetic".?So what are some "common" genetic diseases??This infographic lists ten.?Note that they're not that common compared with non-genetic diseases.?Six of them do have genetic therapy trials.?The rest have drug trials, so they're not being ignored.?These are the ones that don't have genetic therapy trials listed:

Down Syndrome.?Usually trisomy 21.?Excising an entire extra chromosome is likely beyond current gene therapy capabilities.?Another cause is translocation.?In that case, half a chromosome would have to be excised.
Fragile-X Syndrome.?Triplet repeat of variable length.?Variability likely complicates gene therapy.?For instance, how to excise a repeat that is too long vs a repeat that is normal?
Huntington Disease.?Triplet repeat of variable length.?There is also a psychosocial issue where many with Huntington do not want to know their status.?How do you treat a population that is in denial??There is a documentary about it.
Angelman Syndrome.?Caused by uniparental disomy.?Two copies of chromosome 15 are inherited from the father.?The extra copy would have to be replaced with a chromosome from the mother.