In situ hybridization was performed with sections obtained from mammary gland at 10 days of lactation and at 1 and 3 days of involution using either digoxigenin-labeled antisense or sense RNA probe in order to localize expression of WDNM1 and ferritin heavy chain mRNA. The WDNM1 gene was predominantly expressed in the layer of secretory epithelial cells surrounding the lumen of mammary gland alveoli. The lower levels of WDNM1 mRNA were observed at involution day 3 compared to involution day 1. The expression of ferritin heavy chain mRNA also appears to be confined to the epithelial layer of mammary alveoli. The lower levels of ferritin heavy chain mRNA were observed at involution day 3 compared to involution day 1.