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The major surface protein (MSP) 1a of the ehrlichial cattle pathogen Anaplasma marginale, encoded by the single-copy gene msp1α, has been shown to have a neutralization-sensitive epitope and to be an adhesin for bovine erythrocytes and tick cells. msp1α has been found to be a stable genetic marker for the identification of geographic isolates of A. marginale throughout development in acutely and persistently infected cattle and in ticks. The molecular weight of MSP1a varies among geographic isolates of A. marginale because of a varying number of tandemly repeated peptides of 28–29 amino acids. Variation in the sequence of the tandem repeats occurs within and among isolates, and may have resulted from evolutionary pressures exerted by ligand–receptor and host–parasite interactions. These repeated sequences include markers for tick transmissibility that may be important in the identification of …