OPRM1 and Huntington disease: Altogether, these findings for the anterior caudoputamen suggest that the increased intensity of striosomal MOR1 immunostaining began to be detectable by 3 months of age in the Q175KI model of HD and was saturated by 12 months of age, whereas the MOR1 intensity in the matrix began to increase noticeably only after 12 months of age.