The NM23 gene is a conspicuous metastasis-suppressor gene. Eight human genes of the NM23/nucleoside diphosphate kinase family have been discovered. From our large cDNA cloning and sequencing project, we cloned a different transcript ( NM23-H1B) of human NM23-H1 from 18-week-old human fetal brain. The 987-bp cDNA encodes a protein of 177 amino acid residues. Compared with NM23H1, the cDNA contained an additional NH(2)-terminal region (25 amino acid residues). It was mapped to chromosome 17q21.3 using bioinformatics analysis, which shows that the second exon does not exist in NM23-H1. The expression pattern of NM23-H1B showed that it was ubiquitously expressed in normal tissues (15 tissues except colon) at different levels. Our data also indicated that the expression of the transcript in tumors related to tumor differentiation: in poorly differentiated breast carcinoma GI-101, pancreatic adenocarcinoma GI-103, and undifferentiated ovarian carcinoma GI-102, there was no expression. In poorly differentiated lung carcinoma LX-1, lung carcinoma GI-117, the expression level was very low. The transcript band in well-differentiated colon adenocarcinoma CX-1 was significantly higher than that in poorly differentiated colon adenocarcinoma GI-112. A high transcription level was also found in grade IV prostatic adenocarcinoma PC3.