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Abstract This work aimed to deternine whether nitrous acid has some carcinogenic effect or it is only mutagenic i n the f i s s i o n yeast Schizosaccharornyces pornbe. -_a W_ - I rrs a T It also attempted to investigate i f there is any effect for he genetic backgrounds of the different i s o l a t e s on mutations and t h e i r charac-teristics. The r e s u l t s obtained showed that : 1. Different i s o l a t e s gave different total number of inutants with different cliaracteris-Lics. 2, The v a r i a b i l i t y of xx-kants and tl.1ei.r kinds were also noticed spontaneously among the differen* isol a t e s . 3 e Increasing the period of incubation with nitrous acid a t 30•‹C increased the number of mutants, This increase was different f o r the dilfenxtt i s o l a t e s i n the different kinds of mutants, 4. >Titrous acid treatment f o r 3 days showed higher percentages i n the number of leaky and rsverJ~ed nutants. |