. and Chowdhary, B. P. 1998. Fiber-FISH analysis of the 3'-terminal region of the human L-type Ca'+ channel alC subunit gene. -Hwtditus 129: 169-175. Lund, Sweden. lSSN 0018-0661. Received May 4, 1998. Accepted November 8, 1998 Human L-type Ca'+ channel a,, subunit gene (CACNLIAI) maps to the distal region of chromosome 1 2~1 3 .and is composed of z 50 exons spanning over 150 kb of the human genome as estimated by restriction map analysis. However, the structure and the total length of the 3'-end of the gene is not clear because the size of several big introns remains unknown. Here the fiber-FISH technique was used to determine the relative order and size of eight partial genomic DNA clones from the central and 3'-terminal regions of CACNLIAI. The total physical distance of this region, including the size and gap distances between the clones were re-estimated. The results show that the physical order of the tested clones was 5'-g14-5 > g12-2 > g10-8 > g4-5 > g16-7 > g8-3 > g12-5 > g6-20 -3'. Their individual sizes vary between 6.7 and 21.9 kb. Clones g6-20 and g12-5, both containing repetitive exon 45;46-Iike element, were found to be located within 59.1 kb downstream of g8-3 containing earlier identified polyadenylation site, i t . 229.5 kb away from clone g14-5 (exons 10. 1 I ) . The possible implications of this structural complexity is discussed.Wan-SlimgLiu, Deptirtirient of' Generics, Uppsuliz Unirersiry, Bou sheng.liu@genetik.uu.se High resolution visual mapping of stretched DNA by fluorescent in situ hybridization (termed fiber-FISH) has recently been developed (WIEGANT et al. 1992; F~DLEROVA et al. 1994;SENGER et al. 1994;HEISKANEN et al. 1994HEISKANEN et al. , 1995HEISKANEN et al. , 1996. This technique is based on the hybridization of probes to unfixed linearized DNA fibers on glass slide, and has been used to establish the order, orientation and size of several probes (PARRA and WINDLE 1993;HOULEAS et al. 1994;HAAF and WARD 1994;FLORIJN et al. 1995;KLOCKARS et al. 1996;SJOBERG et al. 1997;SHIELS et al. 1997), to rapidly identify aberrations and rearrangements of genes (HEISKANEN et al. 1995) and also to directly visualize the DNA replication (ROSENBERG et al. 1995) and exon mapping (FLORIJN et a]. 1996). In combination with digital image microscopy, fiber-FISH is becoming a powerful tool for high resolution genome mapping.L-type Ca2+ channel is a member of the family of voltage-dependent ion channels. Human L-type Ca2 + channel sllC subunit gene (CACN L I Al) is composed of over 50 exons (SOLDATOV 1994) and is located in the distal region of chromosome 1 2~1 3 (SUN et al. 1992). Recently we found in clones g6-20 and g12-5 a new repetitive element of paired exon 45/46-related sequences and co-localized them to the 3'-terminal part of the a I C subunit gene by fluorescent in situ hybridization technique (SOLDATOV et al. 1998). According to PCR analysis, clone g12-5 overlaps over a 7003, SE-750 07 Uppsiilu, Swtten. E-mail: wanshort distance with g8-3. However, we failed to identify the exact position...