The CCAAT/enhancer binding protein α (C/EBPα) protein is essential for proper lung and liver function and granulocytic and adipose tissue differentation. It was hypothesized that abnormalties in C/EBPα function contribute to the development of malignancies in a variety of tissues. To test this, genomic DNA from 408 patient samples and 5 cell lines representing 11 different cancers was screened for mutations in the C/EBPα gene. Two silent polymorphisms termed P1 and P2 were present at frequencies of 13.5% and 2.2%, respectively. Of the12 mutations detected in 10 patients, silent changes were identified in one nonsmall cell lung cancer, one prostate cancer, and one acute myelogenous leukemia (AML) subtype M4. The 9 remaining mutations were detected in 1 of 92 (1.1%) myelodysplastic syndrome (MDS) samples and 6 of 78 (7.7%) AML (AML-M2 and AML-M4) samples. Some mutations truncated the predicted protein with loss of the DNA-binding (basic region) and dimerization (leucine zipper [ZIP]) domains by either deletions or nonsense codons. Also, inframe deletions or insertions in the fork region located between the leucine zipper and basic region, or within the leucine zipper, disrupted the α-helical phase of the bZIP domain. The inframe deletion and insertion mutations abrogated the transcriptional activation function of C/EBPα on the granulocyte colony-stimulating factor receptor promoter. These mutants localized properly to the nucleus, but were unable to bind to the C/EBP site in the promoter and did not possess dominant-negative activity. The mutations in the MDS patient and one AML-M2 patient were biallelic, indicating a loss of C/EBPα function. These results suggest that mutation of C/EBPα is involved in specific subtypes of AML and in MDS, but may occur rarely in other types of leukemias or nonhematologic malignancies.