DNA sequences containing homopurine d(G1-3A)n tracts are known to be capable of 
adopting non-B-DNA conformations. The structural polymorphism of these sequences 
is a direct consequence of the structural properties of the homopurine d(G1-3A)n 
tracts. Depending on the conditions, d(GA)n DNA sequences can form 
antiparallel-and parallel-stranded homoduplexes, multistranded complexes, and 
ordered single-stranded conformations. On the other hand, much less is known 
about the structural properties of d(GGA)n and d(GGGA)n sequences. In this 
paper, we show that d(GGA)n and d(GGGA)n repeats form antiparallel-stranded, 
intramolecular hairpins. Under physiological salt and pH conditions, the thermal 
stability of these hairpin forms is high, showing, at 50 mM NaCl, melting 
temperatures in the range of 40-50 degrees C. The base-pairing interactions 
involved in the formation of the d(GGA)n and d(GGGA)n hairpins are different. 
G.A pairs importantly contribute to the stability of the d(GGA)n hairpins. On 
the other hand, the d(GGGA)n hairpins are stabilized by the formation of G.G and 
A.A, but not G.A pairs. Homopurine d(G1-3A)n tracts are frequently found at 
genomic locations performing specialized chromosomal functions (i.e. telomeres, 
centromeres, and recombination "hot-spots"). The molecular interactions 
described here are relevant for the understanding of the peculiar structural and 
biological properties of DNA sequences containing homopurine tracts.