PCNA and Huntington disease: Other studies using truncated transgenic HD mice (R6/1 or R6/2) with ages between 2 and 5 months have also reported impairments in hippocampal cell proliferation using the endogenous cell cycle markers Ki-67 and PCNA [28], as well as the exogenous marker 5-bromo-2′-deoxyuridine (BrdU) [23, 26–28].