Phosphomannomutase (PMM) catalyzes the conversion of mannose-6-phosphate to mannose-1-phosphate, which is a substrate for the synthesis of GDP-mannose. This nucleotide sugar is then used in the synthesis of dolichol-phosphate-mannose, which is essential for N-linked glycosylation and thus the secretion of several glycoproteins as well as for the synthesis of glycosyl-phosphatidyl-inositol (GPI) anchored proteins. In the yeast Saccharomyces cerevisiae, SEC53, a gene required for viability, encodes PMM. Given the importance of PMM in glycoprotein synthesis, it is surprising that very little is known about the enzyme in higher eukaryotes. Recently, an autosomal recessive human disease, Carbohydrate-deficient glycoprotein syndrome type I (CDGS-I) has been correlated with severely reduced PMM activity. Here we report the isolation of a cDNA encoding human PMM, a protein of 29 kDa that is 55% identical and 66% similar to yeast Sec53p. Northern blot analysis shows a single 1.4 kb transcript that is ubiquitously expressed, although levels vary markedly among tissues. Expression of the human cDNA in a temperature-sensitive mutant sec53 yeast strain confers growth at the restrictive temperature, strongly suggesting that this gene encodes a functional PMM. Finally, when expressed in BHK cells, PMM is localized exclusively to the cytosol corresponding to its localization in yeast.