The chromosome 21 gene RCAN1, encoding a modulator of the calcineurin (CaN) phosphatase, is a candidate gene for contributing to cognitive disability in people with Down syndrome (DS; trisomy 21). To develop a physiologically relevant model for studying the biochemistry of RCAN1 and its contribution to DS, we generated bacterial artificial chromosome-transgenic (BAC-Tg) mouse lines containing the human RCAN1 gene with a C-terminal HA-FLAG epitope tag incorporated by recombineering. The BAC-Tg was expressed at levels only moderately higher than the native Rcan1 gene; approximately 1.5-fold in RCAN1BAC-Tg1 and 2-fold in RCAN1BAC-Tg2. Affinity purification of the RCAN1 protein complex from brains of these mice revealed a core complex of RCAN1 with calcineurin (CaN), glycogen synthase kinase 3-beta (Gsk3b), and calmodulin, with sub-stoichiometric components including LOC73419. The BAC-Tg mice are fully viable, but long-term synaptic potentiation (LTP) is impaired in proportion to BAC-Tg dosage in hippocampal brain slices from these mice. RCAN1 can act as a tumor suppressor in some systems, but we found that the RCAN1 BAC-Tg did not reduce mammary cancer growth when present at a low copy number in Tp53;WAP-Cre mice. This work establishes a useful mouse model for investigating the biochemistry and dose-dependent functions of the RCAN1 protein in vivo.