Edited by Eric R. Fearon Changes in the O-glycosylation of proteins have long been associated with the development of cancer, but establishing causal relationships between altered glycosylation and cancer progression remains incomplete. In this study, the authors perform comparative analyses of the cellular phenotypes, transcriptional changes, and alterations in the glycoproteome in colon cancer cells that differentially express one glycosyltransferase. Their results provide a wealth of data on which future studies can be based.The search for factors involved in the development and progression of cancer has revealed a multitude of genetic, epigenetic, and phenotypic changes that distinguish malignant cells from their normal counterparts. One such change typically seen in tumors is the alteration of protein glycosylation (1). In particular, changes in mucin-type O-linked glycosylation have been associated with cancer development and poor prognosis-and many diagnostic markers are based on these changes (1). Mucin-type O-linked glycosylation is initiated by a family of enzymes (known as the GalNAcTs 3 or ppGalNAcTs) that catalyze the addition of GalNAc onto the serine or threonine residues of proteins transiting the Golgi apparatus (Fig. 1A) (2). In mammals, other enzymes add additional sugars to the extant GalNAc in a stepwise process to form what are known as core structures, which can then be further elongated and branched by other glycosyltransferases (Fig. 1A) (2). However, in cancerous tissues, these structures are typically truncated, with many existing as only a single GalNAc (Tn antigen) or GalNAc capped with sialic acid (STn antigen) (1). The reasons for O-glycan truncation in cancerous tissues remain largely unknown, although many studies have implicated the enzymes involved in O-glycan extension in these alterations (1,3,4). Likewise, whether the changes in O-glycan structure are contributing to neoplastic transformation or are a by-product of the transformation process itself remains an ongoing area of inquiry.In this issue of JBC, Wandall and colleagues (5) report new insights into the potential role of one member of the enzyme family that initiates O-linked glycosylation (GalNAc-T6) in the cellular transformations typically seen in cancerous cells and tissues. GalNAc-T6 is one of the 20 family members in mammals that are responsible for the initiation of O-linked glycosylation (2). Members of this family are essential in Drosophila and are involved in various cellular and developmental processes (6, 7). Aberrant GALNT6 expression has previously been observed in several cancer types, but the biological role of GALNT6 in vivo is still unknown.To initiate their study, Wandall and colleagues (5) examined the expression profiles for all 20 GALNT isoforms in 288 colon adenocarcinomas and 41 healthy colon samples, observing that GALNT6 expression is largely absent in healthy colon tissue but up-regulated in many colon adenocarcinomas. The authors then used the colon adenocarcinoma cell line LS174T (which...