Background: Renal cell carcinomas (RCC) are predominantly the clear cell (cc) subtype, with a unique biology, characterized by sensitivity to angiogenesis inhibition. However vascular endothelial growth factor-tyrosine kinase inhibitors elicit modest responses in people with rare variant non-clear cell (ncc) RCC. Immune checkpoint inhibitors (ICI) are active against many cancers, but people with rare variant nccRCC have been excluded from frontline trials despite experiencing a more aggressive disease course and poorer prognosis compared to those with ccRCC. UNISON (NCT03177239) aims to test 2 ideas; the activity of ICI in nccRCC, and the novel sequencing strategy of anti-programmed cell death protein (PD)1 immunotherapy, followed by the combination of anti-PD1 and anti-cytotoxic T-lymphocyte-associated protein (CTLA)4 blockade, in people failed by single agent treatment. Methods: This single-arm, two-part trial recruits people of good performance status suffering metastatic or locally advanced unresectable rare variant nccRCC, including but not limited to papillary (type 1/2), chromophobe, sarcomatoid, Xp11 translocation, collecting duct, and unclassified histological subtypes. Participants are offered fixed dose nivolumab at 240mg every two weeks in Part 1 of the trial. If they experience progressive disease, eligible participants may proceed to Part 2 consisting of nivolumab (3mg/kg) plus ipilimumab (1mg/kg) every 3 weeks for up to 4 doses. People experiencing disease control after single-agent or combined ICI are eligible to continue treatment for up to 1 year. UNISON is powered to distinguish a clinically non-relevant objective tumor response rate (OTRR) of 15% in people taking combination ICI whose cancers are refractory to single-agent PD1, versus a clinically-relevant OTRR of 30% at 5% level of significance with 80% power. 85 participants were recruited in Part 1, on the assumption that 55% of those entering Part 1 will be eligible for inclusion in Part 2. Enrolment commenced in November 2017 and was completed ahead of schedule in September 2019. Of the 48 participants projected to experience progression on anti-PD1 immunotherapy, 36 have so far commenced combination ICI in Part 2. Clinical trial information: NCT03177239