Studied social networks and aggressive behavior in school in 2 cohorts of boys and girls in the 4th and 7th grades (N = 695). Measures of social networks yielded convergent findings. Highly aggressive subjects (both boys and girls) did not differ from matched control subjects in terms of social cluster membership or in being isolated or rejected within the social network. Peer cluster analysis and reciprocal "best friend" selections indicated that aggressive subjects tended to affiliate with aggressive peers. Even though highly aggressive children and adolescents were less popular than control subjects in the social network at large, they were equally often identified as being nuclear members of social clusters. Aggressive subjects did not differ from matched control subjects in the number of times they were named by peers as "best friend," nor did the two groups differ in the probability of having friendship choices reciprocated by peers. This research was supported by grants from the Spencer Foundation and the National Institute of Child Health and Human Development (R01 23301).We thank Tamara R. Flinchum and Lynda Ferguson for their assistance in several aspects of this investigation.

Followed 4th-grade subjects (116 girls and 104 boys) in annual assessments of aggressive patterns over 6 years, from childhood through early adolescence. Results indicated that: (a) there were marked normative shifts from childhood to adolescence in the nature of aggressive themes in conflicts, with developmental persistence of direct confrontation and physical attacks (i.e., a "brutality norm") in male-male conflicts and an increase in social aggression and ostracism in female-female conflicts; (b) individual differences in teacher ratings (and in self-ratings) of aggressive patterns were reasonably continuous over 6 years, with a decay in magnitude as the interval between measurements increased; and (c) there were developmental shifts in the factor structure of aggressive measures, along with changes in how measures from the "self" and "others" were aligned. Some theoretical implications for development, continuity, and convergence are discussed.

The nuclear envelope defines a compartment boundary which is penetrated by pores that mediate a remarkable transport process. Precursor RNAs are retained in the nucleus, while processed messenger RNA, transfer RNA and ribosomal subunits are transported to the cytoplasm. Proteins destined for the nucleus become localized soon after synthesis and again following mitosis, while cytoplasmic proteins are excluded. The process is highly specific: a single base change in vertebrate initiator tRNAMet (tRNAiMet) reduces the rate of export 20-fold; a point mutation within the simian virus 40 (SV40) large-T antigen, converting Lys 128 to Thr or Asn, prevents import. Lys 128 lies within a short 'signal' sequence which, when fused to large non-nuclear proteins, causes their accumulation in nuclei. Regions of other eukaryotic proteins also seem to contain nuclear localization signals, although a single consensus sequence has not emerged. We report here that a synthetic peptide containing 10 residues of large-T antigen sequence serves as a nuclear localization signal when cross-linked to bovine serum albumin (BSA) or immunoglobulin G (IgG) and microinjected in Xenopus oocytes. Substitution of Thr at the position of Lys 128 in this peptide renders it six- to sevenfold less effective. The uptake of peptide-linked BSA is saturable, and the rate is diminished by co-injection of free peptide. These findings are indicative of a receptor-mediated uptake process. With the use of anti-peptide antibodies, a family of proteins is revealed in nuclear but not cytoplasmic extracts of human lymphocytes which contain large-T antigen-like sequences.

P-glycoprotein is a highly conserved membrane protein shown to be overexpressed in many multidrugresistant tumor cell lines. P-glycoprotein is encoded by a small gene family in mammalian cells. Class I and II isoforms cause multidrug resistance, whereas class m does not. In this report, we have characterized three P-glycoprotein-specific monoclonal antibodies (mAbs) by high-resolution epitope mapping with a series of hexapeptides. mAb C494 is gene specific, binding to a sequence present only in the class I isoform of hamster and human. The mAb C32 recognizes a sequence conserved in hamster class I and II isoforms but not in class III isoforms. In contrast, the mAb C219 recognizes a highly conserved amino acid sequence found in all P-glycoprotein isoforms characterized to date. These mAbs were used to reveal differential expression and specific localization of the three P-glycoprotein isoforms in hamster tissues by immunohistochemical staining and competition with epitope-specific peptides. Colonic epithelial cells expressed predominantly the class I isoform in a polarized manner, adrenal cortical cells expressed predominantly the class II isoform, whereas a small percentage of skeletal muscle fibers expressed the class III isoform of Pglycoprotein. These findings suggest that the P-glycoprotein isoforms have distinct physiological roles associated with specialized cell functions.The development of multidrug-resistant tumor cells during malignant progression may be a major factor contributing to nonresponse in chemotherapeutic treatment of cancer. The increased expression of the membrane P-glycoprotein (Pgp; Mr, 170,000) is the most consistent change seen in multidrugresistant cells in vitro, and gene transfer studies have shown this change to cause multidrug resistance (MDR) (1-3). The role of Pgp as an energy-dependent efflux pump was proposed from its primary sequence and structural similarity to many membrane-associated transport proteins-most notably, the bacterial transport protein hemolysin B (4-6). The presence of Pgp or its mRNA transcript has been demonstrated in a variety of human malignant tumors, and Pgp may play a role in limiting a patient's response to chemotherapy (7)(8)(9)(10)(11). Pgp is also found in certain normal tissues, including large intestine, adrenal glands, kidney, liver, and brain (12)(13)(14)(15)(16)(17)(18); such localization has led to the speculation that this glycoprotein is involved in normal detoxification and transport of lipophilic molecules.Recent data indicate that Pgp is encoded by a family of three genes in rodent and two genes in human (19). A comparison of the amino acid sequences among the different gene family members, or isoforms, indicates a similar overall structure (20). However, transfection studies that used fulllength cDNAs have suggested that only some Pgp isoforms confer a MDR phenotype on otherwise drug-sensitive cells (19,21). Thus, the class I and II isoforms have been directly implicated in drug resistance, whereas the function of the class III...