We have devised a screen for genes that suppress the loss of contact inhibition elicited by overexpression of the protooncogene MYCN. The initial application of this screen detected nine distinctive suppressors within a representative human cDNA library. One of these genes was ING4, a potential tumor suppressor gene that maps to human chromosome 12p13. Ectopic expression of ING4 suppressed the loss of contact inhibition elicited by either MYCN or MYC but had no direct effect on cellular proliferation. Pursuing the possibility that ING4 might be a tumor suppressor gene, we found inactivating mutations in ING4 transcripts from various human cancer cell lines. In addition, we used comparative genomic hybridization to detect deletion of the ING4 locus in 10-20% of human breast cancer cell lines and primary breast tumors. Ectopic expression of ING4 attenuated the growth of T47D human breast cancer cells in soft agar. We conclude that ING4 is a strong candidate as a tumor suppressor gene.