HTT and Huntington disease: Consistent with this observation in human HD, mutant Htt carriers have a smaller intracranial adult brain volume before the onset of the disease than do controls, probably reflecting abnormal development [15]; indeed, the brain atrophy was evident in zQ175 mice, even before the motor deficits were detected, suggesting that abnormal brain development may also have contributed to the brain volume change, particularly in the early disease phase, similar to human HD.