The gene responsible for neurofibromatosis type 1 (NF1), one of the more common inherited human disorders, was identified recently, and segments of it were cloned. Two translocation breakpoints that interrupt the NF1 gene in NF1 patients flank a 60-kb segment of DNA that contains the EV12A locus (previously reported as the EV12 locus), the human homolog of a mouse gene, Evi-2A, implicated in retrovirus-induced murine myeloid tumors. EVI2A lies within an intron of the NF1 gene and is transcribed from telomere toward centromere, opposite to the direction of transcription of the NF1 gene. Here we describe a second locus, EVI2B, also located between the two NF1 translocation breakpoints. Full-length cDNAs from the EV12B locus detect a 2.1-kb transcript in bone marrow, peripheral blood mononuclear cells, and fibroblasts. Sequencing studies predict an EV12B protein of 448 amino acids that is proline-rich and contains an N-terminal signal peptide, an extracellular domain with four potential glycosylation sites, a single hydrophobic transmembrane domain, and a cytoplasmic hydrophilic domain. At the level of genomic DNA the EV12B locus lies within the same intron of the NF1 gene as EV12A and contains a 57-bp 5' exon that is noncoding, an 8-kb intron, and a 2078-bp 3' exon that includes the entire open reading frame. EV12B is transcribed in the same direction as EV12A; its 5' exon lies only 4 kb downstream from the 3' exon of the EV12A locus. In the mouse the 5' exon of the homologous gene, Evi-2B, lies approximately 2.8 kb from the 3' end of Evi-2A, in the midst of a cluster of viral integration sites identified in retrovirus-induced myeloid tumors; thus, Evi-2B may function as an oncogene in these tumors.