[123I]-N-fluoropropyl-2b-carbomethoxy-3b-(4-iodophenyl) nortropane (123I-FP-CIT) single photon emission computed tomography (SPECT) visualizes the dopamine transporter (DAT) and is useful for diagnosing Parkinson’s disease (PD) and dementia with Lewy bodies (DLB) [1–3]. The gene discussed is SLC6A3; the disease is Lewy body dementia.