You will be shown biomedical passages and your task is to the answer the provided question. Provide a concise and accurate response and do not include any additional text in the response other than the answer.

Question: What is the genetic basis of Rubinstein-Taybi syndrome?

Context: The Rubinstein-Taybi syndrome (RTS) is a well-defined syndrome with facial abnormalities, broad thumbs, broad big toes and mental retardation as the main clinical features
Many patients with RTS have been shown to have breakpoints in, and microdeletions of, chromosome 16p13.3
these breakpoints are restricted to a region that contains the gene for the human CREB binding protein (CBP), a nuclear protein participating as a co-activator in cyclic-AMP-regulated gene expression
RTS was shown to be associated with disruption of the CREB-binding protein gene CBP (CREBBP), either by gross chromosomal rearrangements or by point mutations
Genetic heterogeneity in Rubinstein-Taybi syndrome: mutations in both the CBP and EP300 genes cause disease
A certain level of CREB-binding protein is essential for normal development, since inactivation of one allele causes Rubinstein-Taybi syndrome (RSTS)
In 92 patients, we were able to identify a total of 36 mutations in CBP
We extended the search for mutations to the EP300 gene and showed that mutations in EP300 also cause this disorder.
Mutations in the CREBBP (CREB-binding protein gene) cause Rubinstein-Taybi syndrome (RSTS).
Heterozygous CREBBP mutations were identified in 12 of the 21 patients: five frameshift mutations, three nonsense mutations, two splice-site mutations, and two missense mutations.
It could be possible that genetic heterogeneity is related with novel mutations in other genes.
identified a novel CREBBP missense mutation, c.2728A > G (predicting p.Thr910Ala)
The p.Thr910Ala variant is outside the crucial histone acetyltransferase domain, and this may explain the mild and variable phenotype
Rubinstein-Taybi syndrome (RSTS), a developmental disorder comprising abnormalities that include mental retardation, an unusual facial appearance, broad thumbs and big toes is frequently associated with molecular lesions in the CREB-binding protein gene, CREBBP
Direct sequencing of CREBBP performed in 13 RSTS patients identified the three zinc fingers (CH1, CH2, CH3) and HAT domain as mutational hotspots in which ten novel pathogenic mutations were localized
Mutations or/and deletions in the genes of the cAMP-response element binding protein-BP (CREBBP) (50-60% of the cases) and of the homologous gene E1A-binding protein (EP300) at 22q13 (5%).