INTRODUCTION The vertebrate hindbrain became a focus of attention for devel- opmental biologists when studies of HOM/Hox gene expression revealed startling similarities between its antero- posterior patterning and that of the insect body axis (Graham et al., 1989; Duboule and Dollé et al., 1989; Lumsden and Keynes, 1989; Wilkinson et al., 1989b; McGinnis and Krumlauf, 1992). A number of other genes are now known to be expressed in the hindbrain in patterns suggesting that they act in conjunction with the Hox genes as part of a control system that organises the development of this part of the body. The task now is to understand how the control system operates: which genes regulate which others, and how do they govern the anatomical development? For this, we need to identify as many as possible of the genes involved and to analyse their mutant phenotypes both in terms of effects on the expression patterns and activities of other genes and in terms of effects on body structure. In this paper, we examine the role of the kreisler gene. kreisler, an X-ray-induced recessive mutation, was first described by Hertwig (1944). The homozygous adult is normal in outward appearance, but easily identified by its deafness and abnormal circling behaviour, a consequence of gross malfor- mation of its inner ears. There have been several subsequent studies of kreisler development (Ruben, 1973; Van De Water and Ruben, 1974; Li, 1979). In particular, Deol (1964) reported that in the kreisler embryo the earliest abnormalities are to be seen, concurrently, in the positioning of the otic placodes and in the segmentation of the hindbrain. In this paper, it is the abnormalities of the hindbrain that particularly concern us; for Deol’s detailed description suggested that these might reflect a fundamental disturbance of the mechanisms controlling anteroposterior specification. This hypothesis has been investigated by Frohman et al. (1993), who have analysed the kreisler hindbrain in terms of the expression of five genes – Hoxb-1, Hoxb-3, Hoxb-4, Krox- 20 and FGF-3. They report a complex pattern of abnormali- ties, in which certain regions of the hindbrain express combi- nations of genes that are never normally encountered. We have independently analysed kreisler using some of the same markers and also several others, including neuroanatomical features not studied by Frohman et al. (1993), and have come to a different and simpler view of the homozygous kreisler phenotype. We find that the major abnormalities are co- ordinated in a simple way and can be summed up by saying that two rhombomeres, r5 and r6, are missing. We have looked for cell death in the kreisler hindbrain that might explain their 2199 Development 120, 2199-2211 (1994) Printed in Great Britain © The Company of Biologists Limited 1994 kreisler is a recessive mutation resulting in gross malfor- mation of the inner ear of homozygous mice. The defects in the inner ear are related to abnormalities in the hindbrain of the embryo, adjacent to the ear rudiments. At E9.5, the neural tube posterior to the boundary between the third and fourth rhombomeres, r3 and r4, appears unsegmented, and the region that would normally correspond to r4 is unusually thick-walled and contains many dying cells. The absence of morphological segmentation in the posterior hindbrain corresponds to an altered pattern of gene expression in that region, with major abnormalities posterior to the r4/5 boundary and minor abnormalities anterior to it. From the expression patterns at E9.5 of Krox- 20, Hoxb-1 (Hox 2.9), Hoxb-2 (Hox 2.8), Hoxa-3 (Hox 1.5), Hoxd-4 (Hox 4.2) and cellular retinoic-acid binding protein I (CRABP I), it appears that the fundamental defect is a loss of r5 and r6. Correspondingly, the glossopharyngeal ganglion and nerve, associated with r6 are missing and the abducens nerve, which originates from r5 and r6, is also absent. Examination of Krox-20 expression at stages as early as E8.5 indicates that Krox-20 fails ever to be expressed in its r5 domain in the homozygous kreisler mutant. The abnormal amount of cell death is seen only later. An interpretation is that the cells that would normally become specified at an early stage as r5 and r6 adopt an r4 character instead, producing an excess of r4 cells that is disposed of subsequently by cell death. Key words: kreisler, mouse mutant, rhombomere, cranial nerves, Hox genes, Krox-20, CRABP I SUMMARY The kreisler mouse: a hindbrain segmentation mutant that lacks two rhombomeres Ian J. McKay 1,2 , Ian Muchamore 3 , Robb Krumlauf 3 , Malcolm Maden 4 , Andrew Lumsden 2 and Julian Lewis 1 1 Imperial Cancer Research Fund Developmental Biology Unit, Department of Zoology, Oxford University, Oxford, OX1 3PS, UK 2 Division of Anatomy and Cell Biology, United Medical and Dental School, Guy’s Hospital, London SE1 9RT, UK 3 National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK 4 Developmental Biology Research Centre, King’s College London, 26-29 Drury Lane, London WC2B 5RL, UK