This book aims to explore the phenotype and genotype aspects of a family with SCA3. This Book contains eight chapters. Chapter 1 has discussed, in general, what background inspires us to study SCA3 further with the research conducted. We also present the pathomechanism of SCA3 disease and explore any issues related to SCA3 that are worth being investigated in this study. In chapter 2, we obtained three index patients with SCA3 from three different families. All of these patients had the same polyQ stretch length, but their clinical profiles, the progression of their symptoms, and the worsening of their condition over time were considerably different. These symptoms included ataxia, speech problem, peripheral motor and sensory nerve complaint, and cognitive impairment. Patients might present with a wide variety of symptoms and clinical features when they first show symptoms. In chapter 3, we described the clinical profile of 5 subjects from the same family with an ATXN3 mutation with expanded CAG repeat. It showed a broad range of manifestations. All patients with SCA3 had prominent non-ataxia features besides ataxia signs. Subjects with the prodromal stage showed a less severe disease with only nystagmus, the symptom of peripheral nerve abnormalities, and mild CI. Subjects with the ataxic stage of SCA3 had more extensive neurological manifestations, including the cerebellar, extrapyramidal, pyramidal, oculomotor, peripheral nerve symptoms, and mild CI. The non-motor symptoms of SCA3 include peripheral nerve symptoms and cognitive impairment. In chapter 4, we found that the abnormality in the peripheral nerve is common and may present with symptoms but with or without signs of it. Peripheral nerve abnormalities were already shown in the prodromal stage. NCS analysis showed a varied pattern of peripheral nerve involvement, from the axonal and demyelinating lesion in sensory, motor, and motor nerves. Chapter 5 reports the peripheral neuropathy of one patient with SCA3 who experienced peripheral nerve symptoms preceding the gait symptom. However, the sensory examination was normal, but the NCS revealed axonal demyelinating sensory-motor peripheral neuropathy. Chapter 6 discussed cognitive impairment consisting of general and cerebellar cognitive function of SCA3 patients. The result showed that the general cognitive function in SCA3 subjects tended to be lower than normal subjects. CCAS examination showed impairment in executive function (cognitive flexibility), attention (short-term memory), language (verbal fluency), and memory (immediate memory) which is also a part of executive function. In the subjects with SCA3, the cognitive function was more likely affected by the pathological disease process rather than age and education level. The involvement of the brain region that resulted in the abnormality of the motor and cognitive function was shown in brain MRI volumetry findings. Chapter 7 found a decrease in corpus callosum volume and cerebellum lobules I-II, which control motor function, and lobule IX, which controls cognition.