Propionyl-CoA carboxylase (PCC) is a mitochondrial biotin-dependent enzyme composed of an equal number of alpha and beta subunits. Mutations in the PCCA (alpha subunit) or PCCB (beta subunit) gene can cause the inherited metabolic disease propionic acidemia (PA), which can be life threatening in the neonatal period. Lack of data on the genomic structure of PCCB has been a significant impediment to full characterization of PCCB mutant chromosomes. In this study, we describe the genomic organization of the coding sequence of the human PCCB gene and the characterization of mutations causing PA in a total of 29 unrelated patients-21 from Spain and 8 from Latin America. The implementation of long-distance PCR has allowed us to amplify the regions encompassing the exon/intron boundaries and all the exons. The gene consists of 15 exons of 57-183 bp in size. All splice sites are consistent with the gt/ag rule. The availability of the intron sequences flanking each exon has provided the basis for implementation of screening for mutations in the PCCB gene. A total of 56/58 mutant chromosomes studied have been defined, with a total of 16 different mutations detected. The mutation spectrum includes one insertion/deletion, two insertions, 10 missense mutations, one nonsense mutation, and two splicing defects. Thirteen of these mutations correspond to those not described yet in other populations. The mutation profile found in the chromosomes from the Latin American patients basically resembles that of the Spanish patients.