GRIN1 and phenylketonuria: In contrast to the changes in glutamate receptor expression observed in the BTBR strain (22, 39), adult PKU mice with a C57BL/6N background exhibited reduced GluN1 expression in hippocampal homogenates (Figure 2, A and B) but not in the P2 (synaptosome) fractions (Figure 2, C and D).