Pyruvate carboxylase (PC), a key enzyme for gluconeogenesis and anaplerotic pathways, consists of four domains, namely, biotin carboxylase (BC), carboxyltransferase (CT), pyruvate carboxylase tetramerization (PT), and biotin carboxyl carrier protein (BCCP). PC deficiency is a rare metabolic disorder inherited in an autosomal recessive way. The most severe form (form B) is characterized by neonatal lethal lactic acidosis, whereas patients with form A suffer chronic lactic acidosis with psychomotor retardation. Diagnosis of PC deficiency relies on enzymatic assay and identification of the PC gene mutations. To date, six mutations of the PC gene have been identified. We report nine novel mutations of the PC gene, in five unrelated patients: three being affected with form B, and the others with form A. Three of them were frameshift mutations predicted to introduce a premature termination codon, the remaining ones being five nucleotide substitutions and one in frame deletion. Impact of these mutations on mRNA was assessed by RT-PCR. Evidence for a deleterious effect of the missense mutations was achieved using protein alignments and three-dimensional structural prediction, thanks to our modeling of the human PC structure. Altogether, our data and those previously reported indicate that form B is consistently associated with at least one truncating mutation, mostly lying in CT (C-terminal part) or BCCP domains, whereas form A always results from association of two missense mutations located in BC or CT (N-terminal part) domains. Finally, although most PC mutations are suggested to interfere with biotin metabolism, none of the PC-deficient patients was biotin-responsive.