Q. Dear Editor,
We want to submit a letter to Nature. Is it allowed to cite papers that are submitted but not yet accepted?
A. Dear Prof. Author
Thank you for your enquiry. Yes, it is allowed to cite such papers. If the paper is submitted to a journal and likely to be in the press or published by the time your paper is published (say within a couple of months of your submission), then include the paper in the reference list, with a number. If the work you are citing is less advanced, you should cite it in the text of your paper, in brackets, in the style of (manuscript in preparation) or (personal communication) or similar. These informal refereneces do not go into the reference list of the paper and do not have numbers.