Osteoporosis can be caused by a loss-of-function mutation in the gene for low-density lipoprotein receptor–related protein 5 (LRP5). In this study, the authors, reasoning that a gain-of-function mutation in the same gene might be associated with high bone density, performed biochemical and genetic analyses of a kindred with high bone density, a prominent mandible, and torus palatinus. Genetic analysis revealed an LRP5 mutation, the substitution of valine for glycine at codon 171, that segregated with the clinical findings. In vitro studies demonstrated that the defect in LRP5 resulted in changes in signaling events with other molecules that normally interact with this receptor-related protein, resulting in increased bone density.

Coronary thrombosis and pulmonary embolism are common causes of cardiac arrest, providing a rationale for the use of thrombolytic therapy in cases of cardiac arrest. In this study, patients with cardiac arrest and pulseless electrical activity were randomly assigned to receive tissue plasminogen activator (t-PA) or placebo in a double-blind fashion. Treatment with t-PA did not increase either the rate of survival to hospital discharge or the rate of return of spontaneous circulation.

An outbreak of Serratia marcescens bacteremia among patients in a surgical intensive care unit was extensively investigated. A total of 26 infected patients were identified. In a case–control study, the main risk factors were receipt of continuous fentanyl infusions and exposure to two specific respiratory therapists. Isolates from the patients were similar to isolates from the infused medication.

Despite marked regional variation in the availability of neonatal intensive care, it is not known whether the supply of neonatologists or neonatal intensive care beds is associated with neonatal mortality. This retrospective cohort study involved almost 3.9 million U.S. infants with a birth weight of 500 g or greater who were born in 1995. As compared with infants born in regions with a very low supply of neonatologists (2.7 per 10,000 births), those from regions with a low supply of neonatologists (4.3 per 10,000 births) were less likely to die in the first 27 days of life. However, further increases in the supply of neonatologists were not associated with greater reductions in risk.

Chimerism is the presence of two genetically distinct cell lines in an organism. This report describes a phenotypically normal woman who was found to have tetragametic chimerism after histocompatibility testing of family members suggested that she was not the biologic mother of two of her three children. She had only one cell line in peripheral blood but had more than one in other tissues. Her T lymphocytes showed full tolerance of cells from family members with any combination of four familial HLA haplotypes.