Pallister-Killian syndrome, also known as Teschler-Nicola-Killian syndrome is a genetic condition characterized by mosaicism: 47, XX or XY, i(12)(p10)/ 46, XX or XY. The 47th chromosome is a 12p isochromosome. 12p tetrasomy and hexasomy cases have been described. The disease affects tissues in a differential manner, as most fibroblasts have a defective karyotype, whereas few lymphocytes are affected. This reflects on the clinical traits of the condition.
Routine prenatal ultrasound evaluation may reveal traits such as abnormal presentation close to due date, rhizomelia, diaphragmatic abnormalities such as hernias, anal defects, and heart malformations, especially ventricular septal defect, all of which are highly suggestive of this syndrome.
After birth, the first sign that draws the clinician's attention is hypotonia, that may be severe enough to interfere with respiration and deglutition. Newborns and children also have a specific phenotype, characterized by facial dimorphism: "coarse" facies, high forehead, sparse lashes and eyebrows, shallow supraorbital ridges, hypertelorism, flat nasal bridge, long philtrum , short neck, low-implanted ears, large tongue, hypoplastic nails and small hands, sparse hair and areas of unusual skin pigmentation . The hypotonia persists as the child grows older, leading to difficulties in sitting, standing, and mobility. The clinical picture is completed by intellectual disability, manifested as severe or profound mental deficiency, deafness, seizures, hypoplastic lungs and speaking difficulties. Other abnormalities such as cleft palate, supernumerary nipples, genital malformations, auditory canal stenosis, extra fingers or toes or bald scalp areas may also occur. The neurologic prognosis of these patients is grim, with severely disabled motor capabilities and incontinence  .
Diaphragmatic hernias are considered to be highly specific and are frequent .
These traits may not be present in all affected individuals or may be less prominent. The severity of genotype abnormalities does not correlate with the clinical features of the newborn . In other cases, different sides of the body may be affected unevenly.
The diagnosis relies on the detection of the mosaic tetrasomy of isochromosome 12p. This can be done by various techniques, performed on different cell types. The most reliable choice is to perform an amniocentesis, as i(12p) is found in all amniocytes in fetuses suffering from this condition . Similarly, all newborn bone marrow cells have this trait , whereas a variable percentage of fibroblasts, lung, liver, testes and spleen cells exhibit the abnormality, hence the "tissue-specific mosaicism" term. Lymphocytes are infrequently affected, therefore blood karyotype may not be a reliable method to diagnose this illness, especially due to the high turnover of blood cells. Fibroblasts may be obtained from buccal smears or skin areas with different pigmentation. The percentage of i(12p) cells does not correlate with disease severity . Furthermore, the number of detectable diseased cells decreases as the patient ages .
Fluorescence in situ hybridization (FISH)   and interphase FISH are reliable diagnostic methods . Other methods include karyotype analysis, array comparative genomic hybridization (CGH) and chromosomal microarray. A normal karyotype of blood cells does not exclude the diagnose. In case clinical doubt persists, newer diagnostic methods, such as spectral karyotyping and multiplex-FISH may shed light over the case .