Lectins form a class of proteins that have evolved a specialized carbohydrate-binding function. Based on amino acid sequence analysis, several lectin families have been described and a lectin domain, the (QxW), domain, was discussed recently based on 11 family members. In this paper, the (QxW), domain family is extended to 45 sequences, several of which have very low sequence identity with the previously known members of the family. A hidden Markov model was used to identify the most divergent members of the family. The expanded set of sequences gives us a more complete appreciation of the conserved features, and the lack thereof, in this lectin family. This, in turn, provides new insights in the structural and functional properties of the individual family members.Keywords: carbohydrate binding; hidden Markov model; lectin; (QxW), domain; sequence familyCarbohydrates play prominent roles in many aspects of biology and it is therefore not surprising that many proteins interact with carbohydrates. Proteins or protein domains for which this interaction consists of a pure binding function are normally referred to as lectins. For a recent review on the structure and function of lectins, see Rini (1995).The focus of this paper is the family of lectins whose structure is known from the crystallographic work on ricin (Rutenber & Robertus, 1991) and more recently abrin-a (Tahirov et al., 1995). These two proteins are related plant toxins, consisting of an N-terminal toxic domain followed by two structurally homologous lectin domains. Figure 1 depicts the first lectin domain of ricin. An interesting structural feature of these lectin domains is that they contain three subdomains that are related by pseudothreefold symmetry (different colors have been used for the subdomains in Fig. 1). The pseudo-symmetry in the protein fold is reflected in the primary structure, which contains three internal repeats per lectin domain (Fig. 2).In a recent publication, we have shown that the 100-kDa mosquitocidal toxin of Bacillus sphaericus contains four C-terminal domains that are homologous to the lectin domains of ricin (Hazes & Read, 1995). In addition, we reported the presence of a similar domain in Aspergillus niger a-galactosidase. Ricin-like lectin domains had been recognized previously in other members of the ricin family, viz. abrins and agglutinin (Roberts