Sex in experimental design - MRC

The Medical Research Council (MRC) is committed to funding the best quality medical research, which is relevant to and benefits the whole of society.

MRC requires applicants to embed diversity in the design of their studies (see MRC’s policy on embedding diversity in research design). This includes the requirement for both sexes to be used in the experimental design of grant applications involving animals, and human and animal tissues and cells, unless there is a strong justification for not doing so.

We also expect applications to include information about the sex of the animals used in experiments, as well as the sex of studied tissues and cells. If you don’t know the sex of the cells and tissues you use, you should plan to determine it as part of your research.

The expectations of using both sexes in experimental design were introduced in 2022 and do not apply to existing awards.

Please see the guidance for applicants for additional details.

Use of both sexes is the default

Both sexes of animals, tissues and (non-immortalised) cells should be included as is appropriate for the particular experiment. There is no requirement to ‘balance’ or use equal numbers of both sexes.

This requirement does not apply to the use of immortalised cell lines.

MRC funds research proposals based on the assessment of the quality of the research and value for money in terms of the resources requested. This includes whether or not the funds requested are essential, adequate and justified for the proposed work. Researchers should submit grant costings based on the cost of performing their experiments in both sexes, as appropriate.

Exceptions for single-sex studies with justification

MRC may still fund single-sex studies where there is strong justification in the research proposal for doing so. Cases where the use of a single sex is likely to be appropriate include:

  • where there are acutely scarce resources (for example, human tissue samples of rare diseases)
  • research into the mechanisms of purely molecular interactions (for example, when investigating protein-protein interactions)
  • single-sex mechanisms or diseases (for example, ovarian cancer)

Other reasons for conducting research in a single sex given by applicants will be considered as part of the peer review process. These may include logistical or ethical considerations and should have robust justification.

In most cases, female variability will not be sufficient as a justification for using only one sex. MRC will also not accept as justification that prior work has been performed in only one sex, or that there is a lack of evidence of sex having an effect.

Use the National Centre for the Replacement, Refinement and Reduction of Animals in Research’s Experimental Design Assistant to help you plan your experiments using both sexes.

Community feedback on this requirement

MRC is committed to helping the research community embed consideration of diversity into their research design.

In April 2022, MRC asked for your views on how best to implement these changes, and what support researchers and research teams might need in order to meet the new expectations.

As a result of this feedback, we have been working to make more information available about the change and to ensure the research community receives the necessary support.

More information on our actions and a summary of the community feedback can be found on the UK Research and Innovation engagement hub.

How we developed this requirement

These changes were based on the recommendations of a working group of experts in biomedical research, statistics and experimental design.

Alongside advice from the expert working group, MRC carried out a survey of the research community in 2021.

The scope of the survey covered aspects of diversity (including sex) in health and biomedical research involving:

  • human participants
  • animal subjects
  • in vitro work

It sought to understand current research practices, community perceptions, as well as benefits of and barriers to increasing diversity in research design and conduct.

The survey showed that the majority of MRC researchers saw the benefits of considering diversity in their research. The most frequently cited benefits were increased translatability and reproducibility of research, as well as detecting sex-specific differences.

The most common barriers and concerns mentioned were:

  • the potential increase in the cost of experiments and complexity of research design
  • compliance with the principles of the 3Rs (the reduction, replacement and refinement of the use of animals)
  • commercial availability of samples

Almost all respondents felt there was a need to develop guidance about starting to use both sexes in animal research, and there was a high level of support for funders taking on this role.

Contact us about the requirement


Last updated: 12 December 2023

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