The large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10) is a multifunctional protein that contains a serine-threonine protein kinase (PK) activity (Nelson, J. W., Zhu, J. , Smith, C. C., Kulka, M., and Aurelian, L. (1996) J. Biol. Chem. 271, 17021-17027). Phylogenetic analyses indicated that ICP10 PK belongs to a distinct subfamily of growth factor receptor serine-threonine PKs that are characterized by their ability to function with a limited number of conserved catalytic motifs (Hunter, J. C. R., Smith, C. C., and Aurelian, L. (1995) Int. J. Onc. 7, 515-522). Here, we report the isolation and characterization of a novel gene, designated H11, that contains an open reading frame of 588 nucleotides, which encodes a protein similar to ICP10 PK. The H11 protein has Mn(2+)-dependent serine-threonine-specific PK activity as determined with a GST-H11 fusion protein and by immununocomplex PK/immunoblotting assays of 293 cells transfected with a H11 eukaryotic expression vector. PK activity is ablated by mutation of Lys(113) within the presumtive catalytic motif II (invariant Lys). 293 cells stably transfected with H11 acquire anchorage-independent growth. Endogenous H11 RNA and the H11 phosphoprotein are expressed in melanoma cell lines and primary melanoma tissues at levels higher than in normal melanocytes and in benign nevi. Melanoma cell proliferation is inhibited by treatment with antisense oligonucleotides that inhibit H11 translation, suggesting that H11 expression is associated with cell growth.