correspondence n engl j med 359;24 www.nejm.org december 11, 2008 2617 these two lines of evidence provide support for the role of the NHERF1 variants in the decrease in TmP/GFR values in our patients.Since FGF-23 decreases renal phosphate reabsorption and can induce hypophosphatemia, we studied patients with normal serum FGF-23 concentrations. Rendina et al. hypothesized that serum FGF-23 concentrations should be low in patients with hypophosphatemia. Undetectable FGF-23 levels are considered normal. We did not assess the serum FGF-23 concentrations in our patients according to whether the TmP/GFR values were normal or low, so we do not know whether hypophosphatemia lowered FGF-23 production. Although the serum FGF-23 concentration correlates with the serum phosphate con-The New England Journal of Medicine Downloaded from nejm.org at NYU WASHINGTON SQUARE CAMPUS on May 30, 2015. For personal use only. No other uses without permission.

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Establishing the proportion of fertilized oocytes and early human embryos that proceed to term may help policy makers in their evaluation of when the life of a new human individual begins and in determining the nature of protection to be accorded to it. The rate of spontaneous abortions, although increasing with age, overall does not exceed 15%. However, abortion rates refer only to 'clinical pregnancy', whereas early embryonic loss is more common than generally believed. Evidence of such wastage comes from many sources. Human fecundity rarely exceeds 35% and may be decreasing due to deterioration in semen quality. Embryological studies show that 50% of randomly recovered preimplantation embryos have severe anomalies. The study of sensitive markers of pregnancy, such as human chorionic gonadotrophin, indicates early embryo wastage in the order of 50%. Pregnancy wastage may be a function of the time lapse between ovulation and implantation as the implantation window extends between menstrual cycle days 20 and 24. Finally, data obtained with natural IVF cycles also indicate major losses, with an overall pregnancy rate of 7.2% per cycle and 15.8% per transfer. These data, however, are biased by a high cancellation rate and low oocyte retrieval in natural IVF cycles.

Use of a non-contact infrared laser (IRL) or acid Tyrode's for zona drilling before embryo biopsy was compared by assessing blastomere viability using various fluorescent markers or culture of the single biopsied blastomere, and, by cytoskeletal and molecular cytogenetic analysis of the biopsied embryos following culture to the blastocyst stage. There was no significant difference in the proportion of biopsied embryos that showed no damage in both the biopsied blastomere and in the remaining embryo (acid Tyrode's: 75% versus IRL: 68%), or in the proportion of single biopsied blastomeres that divided in culture (P > 0.05). However, single biopsied blastomeres from laser drilled embryos showed a greater tendency to form miniblastocysts. The proportion of laser or acid Tyrode's biopsied embryos that reached the blastocyst stage by day 6 was similar, although evident earlier (day 5) in the laser biopsied embryos. Spindle abnormalities at the blastocyst stage included tripolar and tetrapolar spindles, but their incidence was not significantly different from controls. In addition, no significant difference was observed in the incidence of chromosomal abnormalities and mosaicism between the two groups. It is concluded that using an IRL at a safe working distance does not cause adverse immediate or longer term effects on the development of human biopsied embryos, although damage can occur if drilling within this distance is unavoidable. Acid Tyrode's drilling can also cause damage, and tended to retard blastocyst development.

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