Identification of a large group of 3′SS that contain Purine-rich sequences between the -3 and -10 positions in front of the 3′AG. A. Diagram showing the location of purine-rich sequences (Pu) between the polypyrimidine tract (Py) and 3′ AG of constitutive exons in the human genome. The U2AF65/35 heterodimer, which normally bind the Py and 3′ AG, respectively, could be disrupted by the presence of Purine sequences between them. B. Percent of A/G at the 3′SS of human constitutive 3′SS (dotted pink line, n = 4620 3′SS) or 3′SS with Pu-rich elements (black solid line, n = 5041 3′SS) as in A. The p values, obtained by hypergeometric test, are 0 for all the nucleotide positions between -10 and -3. C. Percent of A/G shown in the context of individual nucleotides at each position of the 4620 constitutive (Left) or Pu-rich element-harboring (Right) 3′SS.