I wish to perform a PCR-based poly (A) tail length (PAT) assay. After isolating total RNA from cells, I want to ligate their 3' ends in vitro with a 5' phosphorylated DNA adapter. Why do some studies use an oligo with poly (G) while others use one with poly (G/I)? Is G/I-tailing better than just G-tailing? Also, why is it generally G-tailing or G/I-tailing but not anything like C or T?