A novel Arabidopsis thaliana mutant of one member of the PPR (pentatricopeptide repeat) gene family has been identified during the screening of 2500 Arabidopsis individual insertion T2 lines. The deviations in growth and development of the mutant plants appeared already at the initial growth. The germination was completely inhibited (about 30 to 40% of the T3 seeds in each line) or distinctly delayed in comparison with wt plants. The next development of the mutant seedlings varied further on agar medium. Complex phenotypic expression from post-embryonic lethality associated with abnormal pattern of cell division during globular to heart transition to fertile plants with just subtle phenotypic changes was observed within this individual mutant line. The Southern blot and segregation analysis revealed that the mutant plants contained one insert of T-DNA tightly linked to the mutant locus. To be sure that the ambiguous mutant phenotype was due to a mutation at a single locus, we have mapped simultaneously recessive homozygotes without strong embryonic defects and heterozygotes with embryonic defects caused lethality. In both cases, the location of the mutation was assigned on chromosome 1 in position 91.0 ą 4.9 cM in genetic map derived from recombinant inbred lines. To determine the genomic region flanking T-DNA borders we have provided inverse PCR (iPCR) approach followed by sequencing of the products. The sequencing and BLAST search revealed that the insert was integrated into a gene coding for pentatricopeptide repeat (PPR) containing protein At1g53330 which corresponds to the previously mapped position. This gene is composed of 8 tandem arrays of classical PPR motif characterized by 35 amino acid repeats (Lurin et al., 2004). The PPR protein At1g53330.1 was predicted to be targeted to mitochondria by TargetP program, whereas Predotar did not found any targeting signal peptides. Extensive search for other Arabidiopsis mutants affected in expression of particular PPR proteins as well as a more detailed look at the protein action, the affinity to RNA or other protein molecules are currently in progress to elucidate the exact functions of one of the largest and least understood protein families in plants.