Human coronavirus HKU1 (HCoV-HKU1) belongs to coronavirus group II and encodes 16 nonstructural proteins (nsps) which mediate genome replication and transcription. Among these nsps, nsp9 has been shown to possess single-stranded DNA/RNA-binding properties. The gene that encodes HCoV-HKU1 nsp9 was cloned and expressed in Escherichia coli and the protein was subjected to crystallization trials. The crystals diffracted to 2.7 ร… resolution and belonged to space group P21212, with unit-cell parameters a = 83.5, b = 88.4, c = 31.2 ร…, = ฮ’ = = 90ยฐ and two molecules per asymmetric unit. ยฉ 2009 International Union of Crystallography. All right reserved.