Mutant/mutation
In the mutant the wild type csp gene is replaced with a mutated csp gene, lacking region I (region I is a 5-aa sequence at the N terminus of the repeats; see figure below).

Protein (function)
The CS protein is the major protein on the surface of sporozoites and is critical for development of sporozoites within the oocysts and is involved in motility and invasion of both the salivary gland of the mosquito and the liver cells. The protein is also found on the oocyst plasma membrane and on the inner surface of the oocyst capsule. Specific motifs in CS are involved in sporozoite binding to mosquito salivary glands and in sporozoite attachment to heparan sulfate proteoglycans in the liver of the mammalian host. During substrate-dependent locomotion of sporozoites, CS is secreted at the sporozoite anterior pole, translocated along the sporozoite axis and released on the substrate at the sporozoite posterior pole. Following sporozoite invasion of hepatocytes, the CS is released in the host cell cytoplasm.

Phenotype
It has been shown that proteolytic processing of CSP occurs by a cysteine protease during invasion of hepatocytes. Processing occurs on the sporozoite surface (Coppi et al., 2005, J. Exp. Med. 201, 27-33).
Phenotype analyses of mutant  ∆RI indicate that proteolytic processing of CSP does not occur in the absence of region I. The normal development of salivary gland sporozoites indicates that processing does not play a significant role in the mosquito host. Mutant sporozoites showed impaired ability to invade hepatocytes in vitro. Liver stages (EEF's) that were formed showed normal morphology suggesting that that CSP cleavage is required specifically for productive invasion and not for downstream processes. Mutant sporozoites showed a comparable reduction in infectivity compared to wild type when inoculated either by the intradermal or intravenous route suggesting that processing of CSP is not required for sporozoite exit from the dermis. 

Additional information
Mutant ΔRI sporozoites expressed normal amounts of CSP^ΔRI and was exported in normal amounts to the sporozoite surface. In contrast to wt CSP, proteolytic processing of CSP^ΔRI was not detectable.

Overall,  the phenotypic analyses suggest that the CSP cleavage site lies within the highly conserved 5-aa sequence called region I and demonstrate that region I is required for rapid and complete processing of CSP. The phenotype of ΔRI sporozoites  establishes the link between CSP cleavage and hepatocyte invasion and demonstrates that efficient cleavage requires region I.

Moreover, evidence is presented that proteolytic processing of CSP is required for TSR exposure in the mammalian host, i.e., during hepatocyte invasion, but not for its exposure during sporozoite development in the mosquito host. The TSR domain is a known cell-adhesive motif C-terminal to the repeats termed the type I thrombospondin repeat (TSR).

See also mutant ∆Nfull (RMgm-609) in which the wild type csp gene is replaced with a mutated csp gene, lacking the N-terminal region (including Region I) excluding the signal sequence (see figure below)

Figure: Mutated csp genes from the different P. berghei mutant parasites as described in Coppi et al., 2011, J. Exp. Med. 208, 341-56. (RCon: wild type csp; ∆Nfull: see RMgm-609)

Other mutants
RMgm-609: A mutant in which the wild type csp gene is replaced with a mutated csp gene, lacking lacking the N-terminal region (including Region I) excluding the signal sequence