Table 1.

Attributes and levels included in the DCE

Attribute	Level	Explanation for the participant
Mode and frequency of administration		The way the drug enters the body and how many times the drug should be given.
	Single operation *	You will be under general anesthesia (in a deep sleep) during the operation. The operation is one-time with a permanent effect. The drug is introduced into the brain through an injection.
	Lumbar puncture 12 times per year	A lumbar puncture is an injection in the lower back. During this treatment you are awake and the skin can be made numb locally. A lumbar puncture has a temporary effect and must therefore be repeated every month.
	Lumbar puncture 6 times per year	A lumbar puncture is an injection in the lower back. During this treatment you are awake and the skin can be made numb locally. A lumbar puncture has a temporary effect and must therefore be repeated every two months.
Chance of a beneficial effect		The number of people that experience a good result, such as slowing down disease progression. The exact chance is currently not known, therefore this chance is hypothetical.
	20%	20 in 100 persons experienced a good result.
	40%	40 in 100 persons experienced a good result.
	60%	60 in 100 persons experienced a good result.
Risks		The percentage of people that experience a negative side effect.
	1% risk of infection, bleeding, paralysis *	Short-term side effects that can arise immediately after the treatment. There is a 1% risk (1 in 100 persons) of side effects such as infection, bleeding, or even paralysis or death. These side effects can cause permanent damage.
	10% risk of headache, pain at injection site	Short-term side effects that can arise immediately after the treatment. There is a 10% (10 in 100 persons) risk of side effects such as headache or pain on the injection site. These side effects will pass.
	Unknown on long-term	Long-term side effects that occur later, for example after years. The long-term side effects of genetic interventions are currently not known. It is also not known how likely these are to occur. Possible risks that can occur are for example undesirable effects of the injection of genetic material into the brain.
Follow-up		The healthcare provider and hospital that will conduct the follow-up appointments during the treatment period. Please note: this question is about follow-up appointments. A possible operation will always take place in the nationwide expert center for SCA or HD.
	Neurologist in local hospital without expertise *	A neurologist who does not have specific knowledge of SCA or HD, working in the nearest local hospital.
	Neurologist in nearest university hospital without expertise	A neurologist who does not have specific knowledge of SCA or HD, working in the nearest university hospital.
	Neurologist in nationwide expert center (University hospital)	A neurologist who is familiar with SCA or HD, working in the nationwide expert center for SCA or HD. This is a university hospital.
	Nurse practitioner in nationwide expert center (University hospital)	A nurse practitioner who is familiar with SCA or HD, working in the nationwide expert center for SCA or HD. This is a university hospital.

^* Level of the attribute which was used as the reference level for dummy coding