Only the serum-resistant bloodstream forms of Trypanosoma brucei rhodesiense express the serum resistance associated (SRA) protein.

The Trypanosoma brucei species consists of three subspecies. T. b. rhodesiense and T. b. gambiense are human infective forms, while T. b. brucei is lysed upon exposure to the cytotoxic factor in normal human serum. T. b. rhodesiense can however occur as a serum-resistant (R) and as a serum-sensitive form (S). In a study of the molecular basis of serum resistance in T. b. rhodesiense it was shown that in the cloned ETaR1-repertoire only the serum-resistant variants express specific transcripts, encoding a protein with VSG-characteristics. When a serial of freshly isolated T. b. rhodesiense isolates was tested, the presence of the R-transcripts in the serum-resistant populations was confirmed. The absence of the serum resistance associated (SRA) mRNAs in several isolates from T. b. brucei and especially T. b. gambiense (including an a-typical one), indicates that more than one mechanism might be involved in the phenomenon of serum resistance in the T. brucei group. Neither T. evansi, nor T. equiperdum expresses the SRA-transcripts. Interestingly, the transcription of the R-specific transcripts is not maintained during the life cycle of T. b. rhodesiense; in the procyclic forms the SRA-mRNAs are no longer present.