text: what is amyloid beta in alzehimer's disease? how to define amyloid protein and amyloid beta peptide what does 1-40 or 1-42 means in aβ1-40 aβ1-42 means senile plaques vary in protein composition, both in terms of the types of abeta peptides and of other proteins found in the plaques. monomeric abeta peptides are 39 to 43 amino acids in length and occur in two major forms: abeta1-42 and abeta1-40.  the n terminus of abeta peptides is known to be heterogeneous. in senile plaques, the predominant n-terminal-truncated abeta peptides are abeta3-40/42, abeta11-40/42, and abeta17-40/42.