Mutant/mutation
In the mutant the wild type csp is replaced with a mutated csp containing a specific deletion of entire N terminus, (including region I) and  excluding the signal sequence (amino acids NKSIQAQRNLNELCYNEGNDNKLYHVLNSKNGKIYNRNTVNRLLADAPEGKKNEKKNEKIERNNKLKQP)

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 a mutant lacking region 1, ∆RI, (see figure below and RMgm-308) suggest that the CSP cleavage site lies within region I and demonstrate that region I is required for 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 during hepatocyte invasion, but not for its exposure during sporozoite development in the mosquito. The TSR domain is a known cell-adhesive motif C-terminal to the repeats termed the type I thrombospondin repeat (TSR).
In the ∆Nfull sporozoites only 'cleaved' CSP is expressed. Mutant  sporozoites expressed normal amounts of CSP^∆Nfull and was exported in normal amounts to the sporozoite surface. Evidence is presented that the TSR-domain is exposed on the surface of ∆Nfull sporozoites in contrast to wild type sporozoites where the TSR-domain is only exposed after proteolytic processing. Despite increased numbers of sporozoites per oocyst, there was a 10-fold reduction in the number of ∆Nfull sporozoites that reached the salivary glands.These observations suggest that exposure of the TSR on hemolymph sporozoites leads to their nonspecific adhesion throughout the mosquito, explaining the low numbers that reach the salivary glands.
The large differences in infectivity between mutant salivary gland sporozoites that are imoculated via the intradermal or the inrtravenous route and the observations of the migration of mutant sporozoites in the skin suggest that the exposure of the TSR- domain on the ∆Nfull sporozoites enhances sporozoite invasion of hepatocytes but inhibits migration of the sporozoites out of the skin (see also figure below).
 
Additional information

 
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; ∆RI: see RMgm-608)

Figure: A model for the role of CSP in the sporozoite's journey (from: Coppi et al., 2011, J. Exp. Med. 208, 341-56).

Other mutants
RMgm-608: A mutant in which 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).