RPA analysis with pools of cells stably transfected with pPUR/PLP(+)Z or pPUR/PLP(−)Z. Total RNA was isolated from pools of puromycin-resistant cells and incubated with 32P-labeled antisense riboprobes for Plp1-lacZ (which contains Plp1 sequence only from exons 1 and 2) and β-actin. The full-length Plp1-lacZ riboprobe is 387 nt, while the full-length β-actin riboprobe is 304 nt. The fragments protected from digestion by RNases are indicated on the right side, while the position of radiolabeled markers is shown on the left side. Asterisks indicate the position of a protected fragment formed by hybridization with Plp1-lacZ transcripts that contain alternatively spliced exon(s) derived from Plp intron 1 sequence. Lane P contains the full-length (undigested) riboprobes. RNA was isolated from the indicated pools of stably transfected cells (A, TM3; B, +/+ Li; C, N20.1) or untransfected cells (lane U). Pool numbers (I, II, and III) correspond to the same pools used in Table 1.