There is also often vesicoureteral reflux, in which urine passes backwards from the bladder to the ureters, and frequent urinary tract infections.

[3][4][7] The syndrome has a characteristic facial appearance which is similar to that of Kabuki syndrome, including prominent, downward-displaced ears that are underdeveloped, long eyelids, epicanthic folds, a short, broad nose, an open, downturned mouth and a deep groove in the midline of the tongue.

[4] There is sometimes webbing of the neck or bulging eyes, and less commonly there is excessive hair on the forehead or other parts of the body or a unibrow.

[1][3][4] There are typically language and walking delays, and those affected have very low muscle tone and decreased reflexes.

[3][4][8] They may have neural tube defects such as lipomyelomeningocele (a form of spina bifida) or may have syringomyelia (a cyst in the spinal cord).

[4][5] Some of those with the syndrome have been found to have an underdeveloped corpus callosum, the main band of white matter that connects the two cerebral hemispheres.

[4] Those affected may be born with low weight and size[7][8] and may display stunted growth in childhood,[3] although this symptom has been variable and not in every individual with Okamoto syndrome.

A mutation in one of the two copies of HNRNPK results in defects in DNA transcription, and therefore some developmental processes are disrupted or not completed.

[4] Deletions in the region encompassing HNRNPK have been found in the cells of acute myeloid leukemia in approximately 2% of cases.

Atrial or ventricular septal defects are usually treated with observation but can be surgically corrected in severe cases.

However, there is a slight possibility (around 1%) due to germline mosaicism, a phenomenon in which some sperm cell precursors have the mutation and others don't.

Both infants had congenital hydronephrosis due to ureteropelvic junction stenosis, low muscle tone, developmental delay and characteristic facial features including an open mouth and low-set ears.

[8] Au–Kline syndrome was first described in 2015 by Ping-Yee Billie Au, Antonie D. Kline et al. after mutations in HNRNPK were found in two individuals with similar symptoms at their respective practices in Calgary, Alberta, Canada and Baltimore, Maryland, United States.

The practices were united with each other after both submitted the gene as a candidate to the online service GeneMatcher, which matched them together and allowed them to confirm the syndrome.