The local microenvironment instigates the regulation of mammary tetratricopeptide repeat domain 9A during lactation and involution through local regulation of the activity of estrogen receptor α

► Mammary TTC9A protein level falls during lactation and rises during involution. ► The fall and rise are caused by the factors in the local microenvironment. ► TTC9A is a direct target of ERα. ► Local loss and gain of ERα activity is likely the cause for TTC9A regulation.