Spinocerebellar ataxia (SCA) is a complex group of neurodegenerative disorders [1], [2]. To date, a total of 48 subtypes of SCA have been reported with identification of 35 genes [1], [3], among which short tandem repeat (STR) expansion disorders are the most common，such as SCA3 (ATX3) > SCA1 (ATXN1) > SCA2 (ATX2) > SCA6 (CACNA1A) > SCA7 (ATXN7)> SCA17 (TBP) and DRPLA (ATN) [3], [4]. Patients may exhibit a pure cerebellar phenotype or various combinations of cerebellar deficits and extracerebellar deficits affecting the central/peripheral nervous system, such as pigmentary retinopathy, extrapyramidal movement disorders, pyramidal signs, cortical symptoms (seizures, cognitive impairment/behavioral symptoms), and peripheral neuropathy [3]. Clinically, SCA36 is characterized by cerebellar ataxia, progressive motor neuron dysfunction as well as acoustic impairment [5], [6]. In 2011, SCA36 was first reported being inherited in an autosomal dominant mode in Japan due to a pathogenic expansion of non-coding hexanucleotide (GGCCTG)n repeat in intron 1 of the nucleolar protein 56 (NOP56) gene [7]. In normal individuals, the repeat is a polymorphic and complex sequence containing hexanucleotides (AGCCTG), (GGCCTG)n repeat and (CGCCTG), ranging between 3 and 14 units [7], [8]. However, the (GGCCTG)n repeat is highly expanded in SCA36 patients, reaching 6.8 kb ∼18 kb in Southern blotting, which is roughly estimated to be 650 ∼ 2500 repeats [7], [9].

Traditionally, the causal repeat range for a single STR was commonly inferred by a combination of multiple experimental approaches, including capillary electrophoresis, gel electrophoresis, Southern blot analysis[10], [11], [12], [13]. However, it can be very time/money/resource consuming to carry out these methods for determining repeat ranges for thousands of STRs at a genomic scale [11]. Long-read sequencing (LRS) technologies and relevant platforms have paved the way for addressing these limitations, by producing ultra-long reads (>10 kb) and providing better coverage to span long STRs [14], [15]. The combination of preliminary screening methods (Tri-prime PCR, capillary electrophoresis) and low-coverage (∼15 ×) LRS could be successfully exploited to give an exact genetic diagnosis for dynamic mutation disorders, especially when it comes to urgent issues, such as prenatal genetic counseling. In this study, we preliminarily identified one family with SCA36 due to the intronic repeat expansion in NOP56 gene by TP-PCR and capillary electrophoresis, as well as further confirmed the repeat ranges by LRS.