The present invention relates to an (S)-enantiomer of an aminoheteroaryl compound for use in treating and/or preventing cancer in a subject. The invention further relates to a pharmaceutical composition comprising said compound. Another aspect of the invention is directed to an in vitro method for determining the effectiveness of said (S)-enantiomer of an aminoheteroaryl compound, or said pharmaceutical composition, the method comprising the steps of: (a) obtaining a cell or tissue sample from a subject; and (b) determining the subject's NUDT1/MTH1-status; wherein a NUDT1/MTH1-positive cell or tissue sample is indicative of an effective treatment and/or prevention of cancer. In addition, provided herein is a screening method for identifying a target of an (S)-enantiomer of an aminoheteroaryl compound. Furthermore, in context of this invention, the herein described compounds inhibit the biological activity of MTH1.