CREBBP and Rubinstein-Taybi syndrome: Analysis of CREBBP mutations in RSTS patients showed that the mutations were concentrated (44.7% of RSTS patients) in the histone acyltransferase (HAT) domain (295/2442 (12%) of amino acids make up the HAT domain in CREBBP)38, indicating that the HAT domain is crucial for proper CREBBP function.