We have used William Herschel Telescope/INGRID K s images on two high-latitude fields, the Coppi and Groth strips, to obtain galaxy number counts over $180 arcmin 2 , to a depth of K s $ 21:0. Detection efficiency corrections as a function of object size have been calculated on each pointing. We have used a signalto-noise threshold in two complementary half-exposure images to remove spurious detections. Our data cover the range from K s ¼ 14:5 to K s ¼ 21:0, so they are useful for investigating a previously reported change in the number count slope (d log N=dm) at K $ 17. We find a slope b ¼ 0:54 0:63 for K < 17:5 and a slope f ¼ 0:25 0:29 for K > 17:5. A total contribution from galaxies to the extragalactic background light (EBL) in the K band of I ¼ 10:5 nW m À2 sr À1 has been calculated. This K-band EBL coming from galaxies accounts for only $50% of the recent measurements of the diffuse EBL. Standard number count models fail to reproduce the observed slope change at K $ 17:5 unless elliptical and spiral formation is pushed to zd2.

Sister chromatid cohesion mediated by cohesin is essential for accurate chromosome segregation. Classical studies suggest that heterochromatin promotes cohesion, but whether this happens through regulation of cohesin remains to be determined. Heterochromatin protein 1 (HP1) is a major component of heterochromatin. In fission yeast, the HP1 homologue Swi6 interacts with cohesin and is required for proper targeting and/or stabilization of cohesin at the centromeric region. To test whether this pathway is conserved in human cells, we have examined the behavior of cohesin in cells in which the levels of HP1 alpha, beta or gamma (the three HP1 proteins present in mammalian organisms) have been reduced by siRNA. We have also studied the consequences of treating human cells with drugs that change the histone modification profile of heterochromatin and thereby affect HP1 localization. Our results show no evidence for a requirement of HP1 proteins for either loading of bulk cohesin onto chromatin in interphase or retention of cohesin at pericentric heterochromatin in mitosis. However, depletion of HP1gamma leads to defects in mitotic progression.

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