Transcription of the lymphocyte-specific terminal deoxynucleotidyltransferase gene begins at a single nucleotide, but no TATA box is present. We have identified a 17 bp element that is sufficient for accurate basal transcription of this gene both in vitro and in vivo. This motif, the initiator (Inr), contains within itself the transcription start site. Homology to the Inr is found in many TATA-containing genes, and specific mutagenesis influences both the efficiency and accuracy of initiation. Moreover, in the presence of either a TATA box or the SV40 21 bp repeats, a greatly increased level of transcription initiates specifically at the Inr. Thus, the Inr constitutes the simplest functional promoter that has been identified and provides one explanation for how promoters that lack TATA elements direct transcription initiation.