Mutations in TCOF1 and POLR1D cause the autosomal dominant form of Treacher Collins, and mutations in POLR1C cause the autosomal recessive form. TCOF1
TCOF1 is the primary gene associated with TCS, a mutation in this gene being found in 90–95% of the individuals with TCS. However, in some individuals with typical symptoms of TCS, mutations in TCOF1 have not been found. Investigation of the DNA has resulted in the identification of the kind of mutations found in TCOF1. The majority of mutations are small deletions or insertions, though splice site and missense mutations also have been identified.Mutation analysis has unveiled more than 100 disease-causing mutations in TCOF1, which are mostly family-specific mutations. The only recurrent mutation accounts for about 17% of the cases.TCOF1 is found on the 5th chromosome in the 5q32 region. It codes for a relatively simple nucleolar protein called treacle, that is thought to be involved in ribosome assembly. Mutations in TCOF1 lead to haploinsufficiency of the treacle protein. Haploinsufficiency occurs when a diploid organism has only one functional copy of a gene, because the other copy is inactivated by a mutation. The one normal copy of the gene does not produce enough protein, causing disease. Haploinsufficiency of the treacle protein leads to a depletion of the neural crest cell precursor, which leads to a reduced number of crest cells migrating to the first and second pharyngeal arches.
This acronym describes five distinct dysmorphic manifestations, namely orbital asymmetry, mandibular hypoplasia, auricular deformity, nerve development, and soft-tissue disease.Orbital symmetry

O0: normal orbital size, position
O1: abnormal orbital size
O2: abnormal orbital position
O3: abnormal orbital size and positionMandible

M0: normal mandible
M1: small mandible and glenoid fossa with short ramus
M2: ramus short and abnormally shaped2A: glenoid fossa in anatomical acceptable position
2B: Temperomandibular joint inferiorly (TMJ), medially, anteriorly displaced, with severely hypoplastic condyleM3: Complete absence of ramus, glenoid fossa, and TMJEar

E0: normal ear
E1: Minor hypoplasia and cupping with all structures present
E2: Absence of external auditory canal with variable hypoplasia of the auricle
E3: Malposition of the lobule with absent auricle, lobular remnant usually inferior anteriorly displacedFacial nerve

N0: No facial nerve involvement
N1: Upper facial nerve involvement (temporal or zygomatic branches)
N2: Lower facial nerve involvement (buccal, mandibular or cervical)
N3: All branches affectedSoft tissue

S0: No soft tissue or muscle deficiency
S1: Minimal tissue or muscle deficiency
S2: Moderate tissue or muscle deficiency
S3: Severe tissue or muscle deficiency

Radiographs
A few techniques are used to confirm the diagnosis in TCS.An orthopantomogram (OPG) is a panoramic dental X-ray of the upper and lower jaw. It shows a two-dimensional image from ear to ear. Particularly, OPG facilitates an accurate postoperative follow-up and monitoring of bone growth under a mono- or double-distractor treatment. Thereby, some TCS features could be seen on OPG, but better techniques are used to include the whole spectrum of TCS abnormalities instead of showing only the jaw abnormalities.Another method of radiographic evaluation is taking an X-ray image of the whole head.