Given an abstract, generate a keyword (a noun phrase) that best describes the focus or contribution of the paper. Such keywords can be directly from the given abstract or outside it.

Let me give you an example: Abstract: Some patients converted from ventricular fibrillation to organized rhythms by defibrillation - trained ambulance technicians(EMT - Ds) will refibrillate before hospital arrival.The authors analyzed 271 cases of ventricular fibrillation managed by EMT - Ds working without paramedic back - up.Of 111 patients initially converted to organized rhythms, 19(17 % ) refibrillated, 11(58 % ) of whom were reconverted to perfusing rhythms, including nine of 11(82 % ) who had spontaneous pulses prior to refibrillation.Among patients initially converted to organized rhythms, hospital admission rates were lower for patients who refibrillated than for patients who did not(53 % versus 76 % , P = NS), although discharge rates were virtually identical(37 % and 35 % , respectively).Scene - to - hospital transport times were not predictively associated with either the frequency of refibrillation or patient outcome.Defibrillation - trained EMTs can effectively manage refibrillation with additional shocks and are not at a significant disadvantage when paramedic back - up is not available.
The answer to this example can be: Ventricular Fibrillation
Here is why: This term is directly present in the abstract and it is one of the main topic in it. So can be chosen as the medical subject heading.

OK. solve this:
Abstract: Two immunologic subclasses of human pepsinogen I alpha-PG I and beta-PG I, have been identified based on their reactivity toward a murine monoclonal antibody that recognizes an epitope on the alpha-PG I isozymogens. The antibody was used to purify the major alpha- and beta-isozymogens from gastric mucosa and to determine their contributions to the previously described genetic polymorphism of PG I. The alpha-epitope was localized to the pepsin region of the molecules. The two major alpha-PG I isozymogens (Pg 3 alpha and Pg 5 alpha) and the major beta-PG I isozymogen (Pg 4 beta) were demonstrated to contain net charge differences located in the respective pepsin and activation peptide regions. We propose that the alpha- and beta-subclasses contain net charge amino acid substitutions encoded by the corresponding pepsinogen genes: PGA3, PGA4, and PGA5.
Answer:
Amino Acid Sequence