A mouse model of Rubinstein—Taybi syndrome (RTS) was generated by an insertional mutation into the cyclic AMP response element-binding protein (CREB)-binding protein (CBP) gene. Heterozygous CBP-deficient mice, which had truncated CBP protein (residues 1–1084) containing the CREB-binding domain (residues 462–661), showed clinical features of RTS, such as growth retardation (100%), retarded osseous maturation (100%), hypoplastic maxilla with narrow palate (100%), cardiac anomalies (15%) and skeletal abnormalities (7%). Truncated CBP is considered to have been acting during development as a dominant-negative inhibitor to lead to the phenotypes of RTS in mice. Our studies with step-through-type passive avoidance tests and with fear conditioning test showed that mice were deficient in long-term memory (LTM). In contrast, short-term memory (STM) appeared to be normal. These results implicate a crucial role for CBP in mammalian LTM. Our CBP^+/− mice would be an excellent model for the study of the role of CBP in development and memory storage mechanisms.