Transcription start site mapping by RNase protection analysis of human p27Kip1 mRNA. RNase protection assays were performed using p27Kip1 mRNA derived from MCF7 (lane 2), MRC5 (lane 3) or BT20 (lane 4). The intact probe (lane 5) was 535 bases in length including p27Kip1 sequences from 17 to 472 nucleotides upstream of the AUG and 79 nucleotides derived from vector sequences. Molecular size markers are shown in lane 1 and the size of each marker in nucleotides is indicated on the left. The arrow at the right indicates the major protected fragment observed in all three human cell lines. A control reaction using RNA derived from yeast is shown in lane 6.