Determination of the transcription initiation site by primer extension analysis in erythroid and nonerythroid cells. Primer extension was performed as described in Materials and Methods. A same primer-extended product of 175/180 bp was detected with all the poly(A+) RNAs, with a stronger intensity in fetal liver and placenta than in the other tissues. These products were clearly detected in bone marrow and skeletal muscle with an overexposure of the autoradiogram.