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

Helix-loop-helix genes regulate many developmental pathways, and growing ev idence associates dysregulated expression with tumorigenesis. We observed I d-1, Id-2 and Id-3 mRNA expression in proliferating human keratinocytes in vitro with subsequent down-regulation with differentiation. Immunohistochem ical analysis of human tissue sections identified cytoplasmic Id-1 expressi on and nuclear Id-2 and Id-3 expression in the proliferating layers of the epidermis, Furthermore, me observed a columnar pattern of Id-2 and Id-3 sta ining, which may relate to the epidermal proliferative unit. In squamous ce ll carcinoma of the head and neck, Id protein immunoreactivity was observed in the majority of malignant keratinocytes in the most poorly differentiat ed sections, with reduced staining in well-differentiated disease.