There are several humans diseases associated with mutations of PrP. The normal amino acid sequence of the human PrP protein is shown in the figure below highlighted in grey. Mutations are shown below the normal sequence and color coded to agree with the legend in the bottom section of the figure.

Note; the amino acid variance at postition 129 (M or V) is not necessarily associated with the cause of CJD but with the type of expression and possible resistance to certain types of TSE.
However, all 76 vCJD patients examined as of early 2001 are homozygous for M129 (i.e. both genes encode for a Methionine in this position of the protein).
Either, M129 is necessary for transmission of BSE or V129 delays the clinical onset of the disease (thus V129 vCJD patients may not have developed symptoms at this time). In either case, it is interesting that both bovine and ovine PrP have a M at this position.

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