Gall clefydau prin gyfyngu ar fywyd yn ogystal â bod yn fygythiad i fywyd yr unigolyn. Amcangyfrifir bod mwy na 7,000 o glefydau prin ar hyn o bryd, gyda chyflyrau newydd yn cael eu nodi'n barhaus wrth i waith ymchwil ddatblygu. Amcangyfrifir bod clefydau prin yn effeithio ar 170,000 o bobl yng Nghymru, ac amcangyfrifir bod gan 80% o'r cyflyrau hyn ran enetig iddynt. Er bod clefydau prin yn brin yn unigol, maent yn gyffredin ar y cyd, a bydd clefyd prin yn effeithio ar 1 o bob 17 o bobl ar ryw adeg yn ystod ei oes.
Rare diseases can be both life-limiting and life-threatening. It is currently estimated that there are more than 7,000 rare diseases, with new conditions continually being identified as research advances. There are an estimated 170,000 people in Wales affected by a rare disease and 80% of these conditions are estimated to have a genetic component. Although rare diseases are individually rare, they are collectively common, with one in 17 people being affected by a rare disease at some point in their lifetime.