the human bcl-2 gene contains a 39-bp 
 gc - rich region upstream of 
 the p1 promoter that has been shown to be critically involved in the 
 regulation of bcl-2 gene expression . 
 inhibition of bcl-2 expression 
 can decrease cellular proliferation and enhance the efficacy of chemotherapy . 
 here 
 we report the major g - quadruplex formed in the pu39 g - rich strand 
 in this bcl-2 promoter region . 
 the 1245g4 quadruplex adopts a parallel 
 structure with one 13-nt and two 1-nt chain - reversal loops . 
 the 1245g4 
 quadruplex involves four nonsuccessive g - runs , i , ii , iv , v , unlike 
 the previously reported bcl2 midg4 quadruplex formed on the central 
 four g - runs . 
 the parallel 1245g4 quadruplex with the 13-nt loop , unexpectedly , 
 appears to be more stable than the mixed parallel / antiparallel midg4 . 
 parallel - stranded structures with two 1-nt loops and one variable - length 
 middle loop are found to be prevalent in the promoter g - quadruplexes ; 
 the variable middle loop is suggested to determine the specific overall 
 structure and potential ligand recognition site . 
 a limit of 7 nt in 
 loop length is used in all quadruplex - predicting software . 
 thus , the 
 formation and high stability of the 1245g4 quadruplex with a 13-nt 
 loop is significant . 
 the presence of two distinct interchangeable 
 g - quadruplexes in the overlapping region of the bcl-2 promoter is 
 intriguing , suggesting a novel mechanism for gene transcriptional 
 regulation and ligand modulation .