BACKGROUND: Inhibitors of DNA-binding proteins (Id1-4), lacking the basic 
DNA-binding domain, function as dominant inhibitors of cell-cycle regulators. 
Overexpression of Id proteins promotes cancer cell proliferation and resistance 
against apoptosis. Level of Id protein expression, especially of Id1, correlates 
with poor differentiation, enhanced malignant potential and more aggressive 
clinical behaviour of ovarian tumours. Although overexpression of Ids has been 
found and shown to correlate with poor clinical outcome, their inhibition at 
protein level has never been studied.
METHODS: A peptide aptamer, Id1/3-PA7, targeting Id1 and Id3, was isolated from 
a randomised combinatorial expression library using yeast and mammalian 
two-hybrid systems. Id1/3-PA7 was fused, expressed and purified with a 
cell-penetrating protein transduction domain.
RESULTS: Intracellular-delivered Id1/3-PA7 colocalised to Id1 and Id3. It 
induced cell-cycle arrest and apoptosis in ovarian cancer cells ES-2 and PA-1. 
It activated the E-box promoter and increased the expression level of 
cyclin-dependent kinase inhibitor (CDKN2A) in a dose-dependent manner that is 
paralleled by the cleavage of poly-ADP ribose polymerase. These effects were 
counteracted by ectopically overexpressed Id1 and Id3.
CONCLUSION: Id1/3-PA7 could represent an exogenous anti-tumour agent that can 
significantly trigger cell-cycle arrest and apoptosis in ovarian cancer.