To elucidate the mechanism responsible for the absence of p21 (protein) in homeostatic porcine skin and its rapid generation upon injury, we performed a pull-down experiment of Cdkn1a mRNA and its protein binding partners. For this, we utilized a variant of the comprehensive identification of RNA binding proteins by mass spectrometry (ChIRP-MS), which has been successfully used in human cells for large noncoding RNAs, but never before applied to tissues in vivo or to small targets such as Cdkn1a mRNA