Id proteins are dynamically expressed in normal epidermis and dysregulated in squamous cell carcinoma.

Helix-loop-helix genes regulate many developmental pathways, and growing evidence associates dysregulated expression with tumorigenesis. We observed Id-1, Id-2, and Id-3 mRNA expression in proliferating human keratinocytes in vitro with subsequent down-regulation with differentiation. Immunohistochemical analysis of human tissue sections identified cytoplasmic Id-1 expression and nuclear Id-2 and Id-3 expression in the proliferating layers of the epidermis. Furthermore, we observed a columnar pattern of Id-2 and Id-3 staining, which may relate to the epidermal proliferative unit. In squamous cell carcinoma of the head and neck, Id protein immunoreactivity was observed in the majority of malignant keratinocytes in the most poorly differentiated sections, with reduced staining in well-differentiated disease.