Dopamine agonists have also been assessed for ability to slow the rate of progression of imaging parameters of dopamine metabolism or dopamine transporter density, compared with levodopa (Comparison of the Agonist Pramipexole versus Levodopa on Motor Complications of PD [CALM-PD]23, 24 and Ropinirole as Early Therapy versus L-dopa Positron Emission Tomography [REAL-PET]25 studies). The gene discussed is SLC6A3; the disease is Parkinson disease.