We have discovered that the malaria parasite genome encodes two histone fold domain (HFD) containing proteins named ORP1 and ORP2. In higher eukaryotes, the HFD is found in histones and in DNA-binding proteins with a role in transcriptional regulation such as the transcription factor subunits NF-YB and NF-YC. The HFDs of ORP1 and ORP2 are similar to NF-YB and NF-YC, respectively. Surprisingly, in the parasite, the two proteins do not reside in the nucleus but are both found in a little-known protective structure surrounding the oocyst, the capsule. Deletion of either gene encoding ORP1 or ORP2 leads to a block in the rupture of the oocyst; consequently, the sporozoites are not released, and thus transmission through the mosquito is blocked. Structural modelling and in vitro assays supported the hypothesis that ORP1 and ORP2 dimerize via the HFDs. This was verified by a mutant expressing ORP1 (or ORP2) where part of the HFD was deleted; these mutants had the same phenotype as the complete gene deletions. Based on these data, we propose that the rupture of the oocyst capsule is regulated by the dimer formation of ORP1 and ORP2.The identification of ORP1 and ORP2 as essential proteins for mosquito transmission of malaria parasites provides new insights into the elusive oocyst capsule and also suggests that interventions targeting the capsule can be efficient tools for transmission blocking. We have recently focused on the ORP2 gene with the aim to understand the function of the protein outside the HFD. Bioinformatics analysis of ORP2 revealed limited conservation outside the N-terminal HFD when comparing Plasmodium species, and no similar proteins were found in other organisms. We generated deletion mutants lacking the C-terminal parts of the protein, and the results revealed that part of the protein is essential for oocyst rupture, while the most C-terminal region is required for sporozoite motility and cell invasion. Present work aims to understand the role of these proteins in sporozoite motility and invasion.