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Short RNAs regulate gene expression in many species. Some are generated from any double-stranded RNA and degrade complementary RNAs; others are encoded by genes and repress specific mRNAs. Both, it turns out, are processed and handled by similar proteins. These pathways offer a glimpse into a world of small RNAs.

A second case has been found of a nematode gene involved in developmental timing that encodes a short, non-coding RNA. Both RNAs are expressed at specific times and appear to repress target genes by interacting with their 3' untranslated regions. A coincidence? Or does this pathway attract small RNA regulators?

Mutations in the heterochronic gene lin-28 of C. elegans cause precocious development where diverse events specific to the second larval stage are skipped. lin-28 encodes a cytoplasmic protein with a cold shock domain and retroviral-type (CCHC) zinc finger motifs, consistent with a role for LIN-28 in posttranscriptional regulation. The 3'UTR of lin-28 contains a conserved element that is complementary to the 22 nt regulatory RNA product of lin-4 and that resembles seven such elements in the 3'UTR of the heterochronic gene lin-14. Both lin-4 activity and the lin-4-complementary element (LCE) are necessary for stage-specific regulation of lin-28. Deleting the LCE produces a dominant gain-of-function allele that causes a retarded phenotype, indicating that lin-28 activity is a switch that controls choices of stage-specific fates.

Deep vein thrombosis is a serious medical problem that may lead to pulmonary embolus. The authors discuss the noninvasive diagnosis of deep vein thrombosis using venous occlusion plethysmography and ultrasonic duplex scanning.

A case of varices of the entire esophagus in a patient who had undergone resection of a substernal thyroid gland 29 years previously is reported. Dilated esophageal veins, varices, may serve as collateral channels between the portal and systemic systems. When the superior vena cava is obstructed, blood from the upper extremities and head is returned to the heart via downhill varices. Obstruction of the superior vena cava proximal to the azygos vein results in varices of the upper esophagus. Obstruction distal to the asygos vein, as in this patient, results in varices of the entire esophagus. Portacaval shunt has no place in the management of these patients. Downhill varices should be considered in any patient with superior vena caval obstruction from any cause.

The mouse-adapted strain of poliovirus type 2 (Lansing) induces fatal poliomyelitis in mice after intracerebral inoculation, whereas mice inoculated with poliovirus type 1 (Mahoney) show no signs of disease. Previous work indicated that the adaptation to mouse virulence is associated with the viral capsid proteins and that mutations in neutralization antigenic site I of poliovirus reduce neurovirulence of the Lansing strain in mice. The role of antigenic site I in mouse neurovirulence was further explored by constructing an antigenic hybrid virus. Six amino acids in antigenic site I of the Mahoney strain were replaced with a sequence specific for the Lansing strain by using a mutagenesis cartridge. The hybrid virus was neutralized by polyclonal antisera elicited by the type 1 and type 2 strains of poliovirus and by neutralizing monoclonal antibodies directed against antigenic site I of type 2 virus. The hybrid virus induced paralytic disease in mice, an observation demonstrating that a short sequence of amino acids in antigenic site I is an important determinant of poliovirus host range. Antigenic site I may be involved in attachment of poliovirus to cells of the mouse central nervous system.