MADS-box transcription factor genes have been found in nearly all eukaryotes, in plants over 100 genes belonging to this family have been identified. The genes all encode the conserved MADS-box DNA-binding domain. The members of the MADS-box transcription factor gene family are organised into a number of sub-groups depending on their domain structure. A set of downregulated MADS-box encoding genes were identified in a genome wide transcription profiling assay of seeds lacking paternal contribution in the endosperm. Three of the genes in this set: AGL34, AGL36 and AGL90 are very similar and all belonged to the same sub-group. AGL36 has been shown to be maternally imprinted, and has also been shown to interact with AGL90. Very little is known about the third gene: AGL34, and it has been suggested as a possible pseudogene. In this thesis three T-DNA mutant alleles of AGL34 were functionally screened for a seed phenotype. Initially, a seed arrest phenotype was observed in all of the mutant lines. The segregation of the mutant alleles was studied using both resistance selection and genotyping, as one line had lost its selection resistance and another had a low frequency phenotype. The segregation analysis revealed that one of the mutant alleles did not segregate together with its observed seed arrest phenotype, this mutant allele was the only one in which a homozygous offspring was observed. The two other alleles failed to produce homozygous offspring, indicating a recessive homozygous lethal phenotype, additionally a gametophyte defect was also observed in these two mutant alleles. The expression profile of each mutant allele was investigated using qRTPCR, and indicated significantly altered expression of AGL34 only in one mutant line. The two other alleles appeared to have little to no effect on the expression of AGL34. The work presented in this thesis has helped to elucidate the characteristics and functions of AGL34. It has been shown that it is expressed in A. thaliana and involved in seed development, and AGL34can therefore not be considered to be a pseudogene.