Labelled 3-(Methylnitrosamine)propionaldehyde (MNPA) ( M325710). It was detected for the 1st time in the saliva of betel quid chewers and is a potent carcinogen. It exhibits higher cytotoxicity than Nitrosoguvacine (NGC), Nitrosoguvacoline (NG) and 3-(Methylnitrosamino)propionitrile (MNPN).