Hcf101-1 is a high-chlorophyll-fluorescence (hcf) Arabidopsis mutant that lacks photosystem I (1). Photosystem I subunits are synthesized in the mutant but do not assemble into a stable complex. hcf101 was isolated by map-based cloning and encodes an MRP-like protein with a nucleotide-binding domain. The protein is localized in the chloroplast stroma. In green tissue, the Hcf101 level is stimulated by light, and the protein is not detectable in roots. Two independent knock-out lines, hcf101-2 and hcf101-3, are also impaired in Hcf101 accumulation, although to different extents. Like hcf101-1, hcf101-2 and hcf01-3 are hcf mutants with impaired photosystem I. Our results indicate that Hcf101 is a novel component required for photosystem I biosynthesis.