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rubinstein  taybi syndrome ( rsts , omim # 180849 ) was first described in 1963 and is known to be associated with mental impairment , growth deficiency , microcephaly , facial dysmorphia , and skeletal abnormalities 1 .
two genes have been shown to result in this syndrome : crebbp and ep300 
 2 , 3 .
mutations affecting crebbp cause approximately 5060% of cases , whereas mutations of ep300 are associated with only 3% of cases . of note , a recent paper by negri et  al . suggests a higher incidence of ep300 mutations , closer to 8% of cases .
both crebbp and ep300 are transcriptional coactivators and are highly conserved in the expression of many genes involved in embryology , cell growth and differentiation , and tumor suppression 3 .
the second function of these genes is to act as a histone acetyltransferase ( hat ) which opens up the chromatin structure at the locus to be expressed , crucial for overall gene expression 4 .
while the sequences of ep300 and crebbp are more than 70% similar , the two proteins have distinct functions and can not replace one another 5 .
a recent paper expanded the phenotypic spectrum of ep300 mutations to include genitourinary anomalies and a myelomeningocele 7 . yet here , we report a case of a 11yearold girl with very mild physical and cognitive features of rsts and found to have an ep300 mutation on exome sequencing .
the patient is a caucasian female presenting to our genetics office at 9  years of age for short stature , mild dysmorphic features , and mild cognitive impairment .
after an uncomplicated pregnancy , her birth was induced at 34  weeks gestation because of maternal hypertension .
birth weight was 1.8  kg ( 20th centile ) and birth length was 40.6  cm ( 10th centile ) .
her past medical history was significant for an atrial septal defect that never required intervention .
she was diagnosed as an infant with spina bifida occulta due to drainage from a sacral dimple .
she had one pilomatrixoma surgically removed from the upper arm . at the age of 8  years
, her growth was noted to plateau and she crossed from the 50th8th centile within 1  year .
evaluation showed hypothalamicpituitaryadrenal axis suppression , thought to be secondary to exogenous glucocorticoid use in the form of inhaled steroids .
she was prescribed growth hormone with good response . at 11  years 3  months ,
her growth parameters were : weight 38.8  kg ( 55th centile ) , height 145.1  cm ( 45th centile ) , and head circumference , 52  cm ( 25th centile ) .
with regard to development , walking and talking were delayed , both starting at 18  months of age .
she received physical , occupational , and speech therapy by 2  years of age and improved . by the age of 11  years , she had undergone a full battery of neuropsychological testing on several occasions , at several hospitals .
each time , she was found to have overall intelligence in the lowaverage range , with each subtest score in the low average to average ranges .
her physical exam was significant for flat malar eminences with mild ptosis and downslanting palpebral fissures ( fig .  1 ) .
she also had evidence of hypertrophic scarring in several sites from minor trauma ( fig . 
additionally , while the eyebrows are slightly arched , there is no hirsutism . picture  of hands .
note the hypertrophic scarring on the left upper arm from a prior simple laceration that did not require sutures .
the initial genetic workup included a karyotype and chromosomal microarray analysis ( affymetrix cytoscan hd microarray , arup , salt lake city , ut ) .
our next evaluation was whole exome sequencing ( wes ) from a peripheral blood sample submitted to a reference laboratory ( genedx , gaithersburg , md ) using their standard methodology .
this test revealed that the patient was heterozygous for a c.1948dela mutation in the ep300 gene .
this deletion in exon 10 results in a frameshift mutation and a premature stop codon .
this mutation has not been previously reported but is predicted to cause a loss of normal protein function either through truncation or nonsensemediated mrna decay .
we were surprised our testing revealed an ep300 mutation as rsts was never considered in the differential diagnosis . since the father was not tested and this mutation has never been reported ,
however , in retrospect , her short stature , mild cognitive delays , history of pilomatrixoma , and minor facial features fit some aspects of the diagnosis .
even the more subtle findings of maternal hypertension at the end of pregnancy and skin scarring have been reported previously 4 , 6 , 8 , 9 .
like so many have appreciated , mutations in the ep300 can lead to a milder phenotype .
our patient has no notable skeletal findings , lacks hirsutism and microcephaly , and the cognitive delays are quite subtle such that she has a normal iq on repeated testing .
negri et al . hypothesized that a truncated ep300 protein leads to varying phenotypes depending on how severely the protein product is affected 6 .
alterations in the hat domain are proposed to cause the classic rsts phenotype and the cognitive impairment 10 .
our patient 's mild phenotype further confirms this idea that as more of the ep300 protein is preserved , and particularly the hat domain , the milder the phenotype .
again , this case also illustrates that as the prevalence of wes increases , we will likely find that ep300 mutations are more common , and therefore better understand the full range of associated phenotypes .
key clinical messagerubinstein  
 taybi syndrome is associated with intellectual and physical features . 
 crebbp and ep300 are causative . 
 few cases of ep300 mutations are reported . 
 we report a case with mild features of rsts and ep300 mutation on exome sequencing . 
 this illustrates the utility of exome sequencing to expand every genetic phenotype .