Hypotonia, Ataxia, and Delayed Development Syndrome (HADDS)

Hypotonia, ataxia, and delayed development syndrome (HADDS) represents an uncommonly rare disorder that mostly disrupts development of the nervous system. The syndrome is primarily characterized by a range of neurodevelopmental involvements including expressive speech delay, global developmental delay, congenital hypotonia, and cognitive and intellectual impairment. Moreover, facial dysmorphism and genitourinary abnormalities are known as common features outside the nervous system. The clinical and genetic characteristics of this syndrome were first reported by Chao et al. in 2017. It is thought that genetic alterations in the early B cell factor 3 (EBF3) gene are associated with these neurodevelopmental disorders seen in affected individuals. To date, the incidence rate of HADDS is unknown. However, approximately 30–40 individuals have been identified who carry a de novo mutation in the EBF3 gene, achieved by whole exome sequencing technique (Chao et al. 2017; Kim et al. 2020).