- Microcephaly (HP:0000252): Head circumference below 2 standard deviations below the mean for age and gender. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:3322)
- Intellectual disability (HP:0001249): The term intellectual disability or intellectual developmental disorder is used to describe significantly sub-average intellectual and adaptive functioning based on clinical assessment and as measured by individually administered, appropriately normed, standardized and validated tests of intellectual functioning and adaptive behavior, with onset during the developmental period from infancy through adolescence. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:3322)
- Ataxia (HP:0001251): Ataxia refers to impaired coordination of voluntary muscle movement. Cerebellar ataxia refers to ataxia due to dysfunction of the cerebellum. This causes a variety of elementary neurological deficits including asynergy (lack of coordination between muscles, limbs and joints), dysmetria (lack of ability to judge distances that can lead to under- or overshoot in grasping movements), and dysdiadochokinesia (inability to perform rapid movements requiring antagonizing muscle groups to be switched on and off repeatedly). Evidence: TAS. Frequency: Occasional (HP:0040283). (ORPHA:3322)
- Global developmental delay (HP:0001263): A delay in the achievement of motor or mental milestones in the domains of development of a child, including motor skills, speech and language, cognitive skills, and social and emotional skills. This term should only be used to describe children younger than five years of age. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:3322)
- Hyporeflexia (HP:0001265): Reduction of neurologic reflexes such as the knee-jerk reaction. Evidence: TAS. Frequency: Occasional (HP:0040283). (ORPHA:3322)
- Hypertonia (HP:0001276): A condition in which there is increased muscle tone so that arms or legs, for example, are stiff and difficult to move. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:3322)
- Cerebellar hypoplasia (HP:0001321): Cerebellar hypoplasia is a descriptive term implying a cerebellum with a reduced volume, but a normal shape and is stable over time. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:3322)
- Failure to thrive (HP:0001508): Failure to thrive (FTT) refers to a child whose physical growth is substantially below the norm. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:3322)
- Intrauterine growth retardation (HP:0001511): An abnormal restriction of fetal growth with fetal weight below the tenth percentile for gestational age. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:3322)
- Thrombocytopenia (HP:0001873): A reduction in the number of circulating thrombocytes. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:3322)
- Abnormal leukocyte morphology (HP:0001881): An abnormality of leukocytes. Evidence: TAS. Frequency: Occasional (HP:0040283). (ORPHA:3322)
- Anemia (HP:0001903): A reduction in erythrocytes volume or hemoglobin concentration. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:3322)
- Abnormality of coagulation (HP:0001928): An abnormality of the process of blood coagulation. That is, altered ability or inability of the blood to clot. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:3322)
- Ventriculomegaly (HP:0002119): An increase in size of the ventricular system of the brain. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:3322)
- Cerebral cortical atrophy (HP:0002120): Atrophy of the cortex of the cerebrum. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:3322)
- Sparse scalp hair (HP:0002209): Decreased number of hairs per unit area of skin of the scalp. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:3322)
- Premature graying of hair (HP:0002216): Development of gray hair at a younger than normal age. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:3322)
- Cerebral calcification (HP:0002514): The presence of calcium deposition within the cerebrum. Evidence: TAS. Frequency: Occasional (HP:0040283). (ORPHA:3322)
- Neoplasm (HP:0002664): An organ or organ-system abnormality that consists of uncontrolled autonomous cell-proliferation which can occur in any part of the body as a benign or malignant neoplasm (tumor). Evidence: TAS. Frequency: Occasional (HP:0040283). (ORPHA:3322)
- Immunodeficiency (HP:0002721): Failure of the immune system to protect the body adequately from infection, due to the absence or insufficiency of some component process or substance. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:3322)
- Oral leukoplakia (HP:0002745): A thickened white patch on the oral mucosa that cannot be rubbed off. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:3322)
- Short stature (HP:0004322): A height below that which is expected according to age and gender norms. Although there is no universally accepted definition of short stature, many refer to "short stature" as height more than 2 standard deviations below the mean for age and gender (or below the 3rd percentile for age and gender dependent norms). Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:3322)
- Dermal atrophy (HP:0004334): Partial or complete wasting (atrophy) of the skin. Evidence: TAS. Frequency: Very frequent (HP:0040281). (ORPHA:3322)
- Bone marrow hypocellularity (HP:0005528): A reduced number of hematopoietic cells present in the bone marrow relative to marrow fat. Evidence: TAS. Frequency: Occasional (HP:0040283). (ORPHA:3322)
- Excessive wrinkled skin (HP:0007392). Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:3322)
- Generalized hyperpigmentation (HP:0007440). Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:3322)
- Nail dystrophy (HP:0008404): Onychodystrophy (nail dystrophy) refers to nail changes apart from changes of the color (nail dyschromia) and involves partial or complete disruption of the various keratinous layers of the nail plate. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:3322)
- Generalized hypopigmentation of hair (HP:0011358): Reduced pigmentation of hair diffusely. Evidence: TAS. Frequency: Frequent (HP:0040282). (ORPHA:3322)
These phenotypes are associated with the disease Hoyeraal-Hreidarsson syndrome (ORPHA:3322).