A standard numbering scheme for Class A β-lactamases is proposed to facilitate the comparison of molecular studies of different enzymes in this class. β-lactamases catalyze the hydrolysis of β-lactam rings in antibiotics, protecting bacteria from these drugs. These enzymes are classified into classes based on sequence similarity, with Class A being the largest. Class A enzymes are found in both Gram-negative and Gram-positive organisms, and their sequences vary in length, with leader peptides not being homologous. To avoid confusion in comparisons, a standard numbering scheme is introduced, based on alignment of 20 Class A sequences, preserving as much as possible of the numbering used by Ambler for the first four members of the class. The scheme is labeled 'ABL' and is used for homologous residue comparisons. The active site serine residue is assigned ABL number 70, and the numbering starts within some leader sequences. The scheme is not intended to replace the natural numbering for individual proteins but to provide a consistent reference for comparisons. The alignment is based on X-ray crystal structures and may be revised as new data becomes available. The scheme is not definitive, and some residues may not be considered homologous. The scheme is expected to be extended to other proteins, such as Class C β-lactamases, as structural data becomes available. The proposed scheme aims to standardize the numbering of Class A β-lactamases for easier comparison and analysis.A standard numbering scheme for Class A β-lactamases is proposed to facilitate the comparison of molecular studies of different enzymes in this class. β-lactamases catalyze the hydrolysis of β-lactam rings in antibiotics, protecting bacteria from these drugs. These enzymes are classified into classes based on sequence similarity, with Class A being the largest. Class A enzymes are found in both Gram-negative and Gram-positive organisms, and their sequences vary in length, with leader peptides not being homologous. To avoid confusion in comparisons, a standard numbering scheme is introduced, based on alignment of 20 Class A sequences, preserving as much as possible of the numbering used by Ambler for the first four members of the class. The scheme is labeled 'ABL' and is used for homologous residue comparisons. The active site serine residue is assigned ABL number 70, and the numbering starts within some leader sequences. The scheme is not intended to replace the natural numbering for individual proteins but to provide a consistent reference for comparisons. The alignment is based on X-ray crystal structures and may be revised as new data becomes available. The scheme is not definitive, and some residues may not be considered homologous. The scheme is expected to be extended to other proteins, such as Class C β-lactamases, as structural data becomes available. The proposed scheme aims to standardize the numbering of Class A β-lactamases for easier comparison and analysis.