CEBPA is located on chromosome 19q.32. From the CEBPA mRNA, 2 major proteins are generated from 2 distinct AUG start sites, namely, a p42 and a p30 isoform (A). CEBPA may be transcribed from the 2 alleles (allele A: biallelic mutations in CEBPA may occur at the N-terminus or at the C-terminus [C/N mutation]). Patients with a C/N double mutation generate a p30 isoform only from 1 allele (termed allele 1) and basic leucine zipper (bZIP) domain–mutated p42 and p30 isoforms (red) from the other allele (B). Biallelic mutations at the N-terminal (N/N) generate p30 isoforms only and not p42 isoforms (C). Biallelic mutations at the C-terminus (C/C) generate p42/p30 C-terminally mutated isoforms from both alleles, all defective in the bZIP domain (D). CEBPA double mutant leukemias never express a wild-type p42 protein.